The sun is out in northern Berlin. As he walks along a leafy suburban street, Rupert Friend praises the fine weather and the pretty surroundings. Then, with a characteristically abrupt change of topic, he begins to talk about the apocalypse. One caused by emissions or war or an asteroid. The actor has noticed that, instead of scribbling in a notebook, I’m using a digital voice recorder. “When we run out of power, the internet, all that,” he says, “when the whole world has to decide, ‘Right, back to basics!’, you’re going to be fucked with your Dictaphone.” I tell him I don’t know shorthand. “Then I think you should consider a night course,” Friend replies, gravely.

The 33-year-old is dressed in canvas trousers and a worn green jumper. Under combat boots he has on a pair of new, luxurious socks, just stolen from a Guardian photoshoot. He paraphrases something Jerry Hall once said: “only amateurs hand back all the clothes.” The actor’s dark hair is mussed and spiky, grown out now from an all-over shave he underwent last year, when he was cast as the lead in Hitman: Agent 47. In the film, out this month, Friend plays a professional assassin who is perfectly, palely bald. Not a look that particularly flattered his hawkish face. Friend recalls that he would sometimes meet people “and they’d assume I was actually there to kill them”.

So this apocalypse, I say. If I’m going to be in trouble without a voice recorder, what about you? No cameras, no lighting rigs, no TVs. How will you earn a living? Friend giggles. For three years now his main employment has been on the American spy drama Homeland, a show in which he stars alongside Claire Danes and plays a CIA hardman called Peter Quinn. A fifth series is currently filming in Berlin, which is why Friend is here, due on set in a couple of hours for a night shoot. “I suppose,” he says, “I’d have to do plays.”

You don’t sound delighted.

“No.”

I remind him of the actor’s cliche, that his heart belongs to the stage. “Right,” says Friend. “I’m debunking that myth. I don’t love plays. I don’t love doing the same thing, every night, for 100 nights in a row.”

Despite this, he has appeared in some decent theatre. He was kindly received as a tweedy confidence trickster in a Dennis Potter play, Brimstone And Treacle, in 2012. Before that he appeared in The Little Dog Laughed with Tamsin Greig at the Garrick, and was roundly praised. Friend explains: “What theatre people love about theatre – and I totally understand it, I just don’t share it – is that they feel they mint something, afresh, every night.” He prefers filming, “because I would rather do something until I’ve done it and then know it’s done. New day, next thing!”

We walk on, just strolling until he has to get into a car and whizz off to pretend to be CIA. “I guess the reason I wanted to be an actor was that it felt like it would offer something different all the time,” Friend says. “A cliched example, but say I’m an accountant. I go into work and do accounts every day until I die. That seemed very hard to get my head around. Even if you replaced the job with something more exciting, like, I don’t know, being an astronaut. If I was an astronaut I’d want to go to space one week, but then…”

You’d be the guy complaining to Nasa about having to visit the moon a second time. “Exactly. I’ve seen the world from up there already. It’s still big, still blue. It’s still spinning.” Friend hits his hands together in a gesture of impatience. New day, next thing.

“A bit of a nonconformist,” is how Danes describes her Homeland co-star. Emily Blunt, who appeared alongside Friend in the 2009 film The Young Victoria, has spoken of his “loathing for crowd opinions”. He certainly has an original way about him, and might even qualify as an eccentric – that high-pitched giggle all at odds with the forbidding face, his general good cheer struck through with seams of real grouchiness. (On groaning: “To me it’s a very conscious decision that people make. ‘I am now going to make a noise when I stand up out of a chair.’ You don’t need to!”) During our walk he catches himself saying the word “should” too often, and starts to murmur a self-critical monologue. “Look at me with my ‘shoulds’. The Friend Lectures… Come on a walking tour and be told what to think by someone who doesn’t believe in telling people what to think.”

He has written an unpublished collection of short stories, two short films and the lyrics to a very good jazz album, Everything We Hold, in collaboration with the musician Adam Waldmann. When Friend was cast as the lead in Hitman: Agent 47, a movie based on a video game, producers sent him a copy, but the actor – a “technophobe” – just walked his avatar in circles before giving up. When I remind him he’s supposed to be promoting Hitman: Agent 47 and he might want to talk a bit more about it, Friend agrees, rather sadly. Then he says: “You could make all that up. It won’t be as interesting as what we were talking about.”

We had been talking about his lifelong aversion to milk. Friend grew up in Oxfordshire, in a one-pub village he once described as half Dylan Thomas and half Postman Pat. His father was an art historian and his mother a mediator. The generation above were dairy farmers. Friend remembers his grandparents keeping billycans of freshly drawn milk in their house. “It sounds very Cider With Rosie. But it was basically thick, warm cream. And if you try to have that on your cornflakes you will puke, I promise you.”

By the sounds of it he was a peculiar child, “happy and anxious at the same time”, he says. The eccentric streak might have been a hand-me-down from his mother. She was an apostrophe obsessive, Friend says, who would correct bad grammar on the village grocer’s blackboard. He was bullied at school, an experience that left him with a skin thickened to outsiders’ opinions. Into his 30s, Friend retains a tetchy, studentish aversion to agreed conventions of behaviour and politeness, “like that thing we all do,” he says, “where we end a social engagement and even if it hasn’t been that good we promise to do it again. Because we feel so awkward saying: ‘I might not see you again – BYE!’”

After school he was in a band (“called Marrakech – you can guess what we were smoking at the time”) and following a gap year in the Cook Islands, Friend enrolled at Webber Douglas Academy, a drama school in London. “Yes, there’s some technique involved,” he says of acting. “But I think there’s been an overcomplication of it, in a sense. If you say to a kid: ‘We’re going to play cowboys and Indians, that’s the fort, you guard it’ – the kid doesn’t have to sit and think about the role for 20 minutes in silence with their acting coach. They just do it.” That’s how he tries to act, says Friend, through a process of infantalising.

He was in his early 20s, and still at drama school, when he got his first job, appearing alongside Johnny Depp in a 2004 film, The Libertine. Friend initiated a tradition of keeping mementoes from his films, and from The Libertine he took away “the bloody undershirt that I died in, covered in fake gore. I had that hanging in my bedroom for an inordinate amount of time, ridiculously proud of it.” Cast as the solider-seducer Wickham in Pride & Prejudice, he kept a large dragoon’s hat. A set of ornate swords were probably the best thing to come from “a very terrible Roman film I did called The Last Legion” in 2007. A couple of years later, when he’d finished making Chéri with Michelle Pfeiffer, Friend watched on, impressed, as the director Stephen Frears made arrangements to take home an entire belle epoque greenhouse. “He wanted to put it in his garden.”

While making Pride & Prejudice in 2005, Friend began dating Keira Knightley, that film’s lead. It prompted a lot of badgering from the tabloids, scrutiny that lasted the five or six years the pair were together, and Friend became one of those scratchily private actors who resist intrusion even in their contracted publicity engagements. “Because I find the alternative quite sad,” he tells me, “the idea that everything is for sale. That you’ll say, ‘Come into my home and I’ll pretend you’re my best friend, even though you’re a camera crew.’” There were uncomfortable sit-down interviews, and during red-carpet walks beside Knightley he often cut a surly figure. It cannot have helped that his own films during this period were not always brilliant. “Deeply unpleasant” was the Guardian’s verdict on Outlaw (2007), “bizarrely acted” its take on The Kid (2010). A turning point seemed to come with his linchpin appearance as a prison psychologist in Starred Up, an independent drama that triumphed on the festival circuit in 2013.

“I can know the moment I walk on to the set,” Friend says, describing the feeling of being in a bad film. “It’s like stepping on to a boat that pushes out from port and only then you see the leaks pouring out everywhere. You basically tell yourself, ‘Oh right. So I’m baling out water, now, from here to the New World…’” He talks about how rarely the right people come together at the right time. “Less than 1% – that’s how often it happens. Because you can say, ‘This director has won all these Oscars. This writer has won the Nobel prize. These actors are all terrific, too.’ It doesn’t mean that putting them together is going to make a wonderful piece of work.” Successful film-making, he says, “is alchemy. And about as likely, actually, as getting gold from lead.”

In 2009, while promoting Chéri, Friend made an appearance on the daytime US talkshow The View. It was a memorable event for several reasons, not least because the actor was asked to explain live on air what the English mean when they say “fanny”. At one point in the interview Friend admitted, casually, that he didn’t own a television set. He was told it was a shame, as British actors were then finding lots of good work in American TV. He looked polite, and sceptical.

He explains to me that, at the time, he couldn’t think of anything worse than being in a TV show. “I didn’t want to be bound in for things, to know what I was going to be doing the following year.” But by 2012 he was in a rut. His relationship with Knightley had ended the previous year. He’d launched himself on an ambitious building project in east London, trying to turn an old factory into a house. The work ate up all the money he’d made in a decade of acting, and then some. He was appearing in the Dennis Potter play, “earning £300 a week”, when he was asked to audition for Homeland. Friend said no. “And then I thought, ‘Well, hang on. You’re sticking to some philosophy you invented when you were about 22. And it’s not necessarily making you that happy.’”

You weren’t happy?

“Well, I wasn’t fulfilled, no. I’d done a West End play. I’d done a fringe play. I’d done 10 years of films. I was a bit drifty with it all.” He was beginning to think about quitting. “Maybe the job wasn’t right for me and I should stop. I thought, before I can know the answer to that, logically, Socratically, I have to eliminate a few more possibilities.”

Homeland was probably the hottest show going, a ferocious first series having introduced the world to Danes, as CIA agent Carrie Mathison, and Damian Lewis as Nicholas Brody, the possible terrorist she was tracking. Friend’s auditions were remote and obscure, the actor sent lines that he’d speak into a camcorder while standing on the building site that was then his home. His sister spoke Danes’s lines. When he was hired, Friend flew out to the US not knowing who his character was, or “if it would be for a week or for a year. Should I rent a house? Should I rent a car?” Drip-fed scripts, he slowly worked it out. “I knew that my character’s job was going to be to kill Damian Lewis’s character, Brody. But then – and here’s a bit of industry politics for you – Damian wins an Emmy. So the network realises everyone loves him. ‘He’s the best actor on television now! We don’t want him killed off.’”

Poor Quinn. A hitman without anyone to hit.

“I know. I was like, so, what, Peter just goes home now?” Strolling in the Berlin sun, Friend wonders, actually, how the showrunners might have written him out at that point. “‘Hey, where did Peter go?’… ‘Peter didn’t even leave a note’ ”

But Quinn was kept, to striking effect. He put a knife in Brody’s hand during an interrogation and later blasted Carrie with a sniper rifle (while she was pregnant). When Brody was belatedly killed off, the writers turned up the heat on Carrie and Peter’s relationship. The role grew.

More than most dramas, Homeland clings to contemporary global politics, settings and scenarios borrowed only months after leaving the news cycle. I ask Friend what he makes of the uneasy proximity between western intervention abroad, and the making of entertainment about it back home. He thinks for a while and says that art often comes from the process of something happening (“Falling in love, say”), followed by the desire to express what that felt like. In the case of Homeland, “I think maybe this is America’s way of trying to talk about a very odd state of affairs. This whole kind of 6,000-mile policing thing. ‘But if you do it to us, that’s not OK.’”

What about the UK? Between six-month shoots on Homeland, Friend lives in New York. I notice he has started using you, not we, in reference to the British. Are we already so distant? “I don’t know,” he says. “‘We’ is a difficult word for me. I don’t know if I feel ‘we’ about anything.” In New York Friend lives with his fiancee Aimee Mullins, an American whose father is Irish. Friend mentions a particular point of affinity between himself and his future father-in-law, that both of them really miss pubs.

Mullins was once a Paralympic sprinter who competed for the US at the 1996 Games in Atlanta. Born without fibulae, the now 39-year-old’s lower legs were amputated when she was very young. More recently she has worked as a model, for L’Oréal and others, and a public speaker, giving a massively watched TED talk on the science of prosthetics. News of the couple’s engagement filtered out clumsily, in December, when Mullins attended an event organised by L’Oréal and was publicly congratulated from the stage. I ask Friend how his obvious impatience squares with the notion of settling down.

He says: “You’re presuming the person to whom I’m engaged is some sort of concrete block. I’m engaged to somebody with as much wind in her sails as I have... I find that phrase nightmarish, actually, settling down. It implies loss of love of life.”

At this point we happen to walk by a fountain, noisily gushing water. Friend stops on the pavement and embarks on an obscure joke that involves him apologising and making the sound of a fly unzipping. I’m baffled. “So that when you listen to this later,” Friend explains, “on your recorder…”

I’ll think you stopped to take a wee.

“Yes.”

Friend chuckles happily. When our conversation resumes, it is not about his private life, but about his frustration with people who like only cats or dogs, and how he recently found an “interesting wrinkle” on his forehead that reminds him of a double apostrophe. As the studio car pulls up, we murmur something about how this social engagement has been fun, and maybe we should do it again sometime. But in the end we agree we’re unlikely to see each other again – BYE!

• Hitman: Agent 47 is released on 27 August

Styling by Melanie Wilkinson. Grooming by Cathy Skene. Grey shirt, £190, by Acne, from mrporter.com. Red top, £55, cosstores.com. Trousers, £540, by Valentino, from matchesfashion.com. trainers, £160, oliverspencer.co.uk.

• This article was edited on 1 September 2015. An earlier version gave the wrong professions for Rupert Friend’s parents, and stated that his fiancee Aimee’s mother was Irish. Also, Friend does not know Jerry Hall personally, and related her advice secondhand. The hat from Pride & Prejudice, and the swords from Last Legion, were gifts and not taken from the set. Finally, Friend has written unpublished short stories for friends, not a novella.