As a child in Los Angeles, Jason Segel liked puppets and comic-books and dreamed of being a
real-life superhero. Each day he would set off from home wearing a crimson cape beneath his regular clothes, its trailing loose end tucked into his trousers. He did this for years, right into puberty. When his parents asked why he was wearing a cape, he would fix them with a gimlet stare. “Just in case,” he said ominously.

Cometh the hour, cometh Super-Doofus. At the age of 34, Segel still likes puppets and comic-book capers but he has found a way to make it pay. His stock-in-trade is the American man-boy, an amiable accident just waiting to happen. Roles in I Love You Man, Forgetting Sarah Marshall and Jeff, Who Lives at Home have made him rich and famous, Hollywood’s go-to man for grinning ineptitude, with a net worth estimated at around $30m. Everybody likes Segel when he plays the adolescent clown. Everybody, it transpires, except Segel himself.

I meet the actor in Barcelona, a handsome city, full of beggars, where Pharrell’s Happy spills out of every other shop doorway along the Rambla de Catalunya. Segel is in town to promote his role in Sex Tape, a comedy of embarrassment, full of pratfalls and chases. In person, however, he is tall, trim and self-possessed. His hair is neatly cut; his suit tightly tailored. It is the first, visual clue that he has reinvented himself.

“The era of me playing a man-boy who really needs to grow up is gone,” he announces, almost by way of introduction. “Things have changed.”

Full credit to Segel if he wants to expand his repertoire. I’m just not convinced that Sex Tape is quite the best calling-card for the new, improved him – even if he did have a hand in writing the script. Jake Kasdan’s caper is about an everyday suburban couple (Segel and Cameron Diaz) who film their three-hour lovemaking session (all the positions in the Joy of Sex), only for the material to leak on to the web. They’ve got to get it back; their reputations will be ruined. So off they go, bouncing between various addresses, attempting to steal tablets and laptops from their nearest and dearest, squawking and bickering as they go.

I saw the film as a juvenile romp: fitfully funny, eminently disposable. But Segel discusses his work in such endearingly lofty terms that it brings me up short. I half-suspect that he’s joking; I wouldn’t put it past him. He’s talking ethics, morality and the human condition. His ears are cocked to a higher calling. He wants to tilt for the heavens and take the audience with him.

“Just lately I’ve been doing a lot of thinking,” he says. “And it led me to a rabbithole about the function of art. I’m interested in finding the union between all of us. I want to give people the opportunity to say: ‘Oh my gosh, that’s me up there.’ I want to express something they hadn’t been able to put into words.”

I think about Sex Tape. It contains a scene in which Segel loses his trousers and is pursued through the house by an alsatian dog.

“My goal right now is to keep walking through fear,” he continues. “I think everyone has that feeling that they are meant for something more. That there is this form of self-actualisation that has yet to occur. Fear is the only thing that stops us achieving our goals.”

I think about Sex Tape. It contains a scene in which a spluttering Segel falls off a balcony.

“I needed a part where I felt I was being challenged,” he says. “I had to step it up and play a man.” And yet what I liked about Segel’s performance was the sense that his clammy suburban dad is still haunted by the ghosts of his adolescent past. The grownup hat sits awkwardly on his head, in constant danger of falling off. He responds to each fresh crisis with a darting gaze and a ghastly smile that would do Richard Nixon proud.

Had he not gone into comedy, he’d have been a lawyer like his dad. But by his late teens he was already working on TV, playing a genial stoner in Judd Apatow’s short-lived, much-loved Freaks and Geeks. He was purely terrific as Paul Rudd’s bromantic foil in I Love You, Man, voiced a plucky young boffin in Despicable Me and merrily exposed himself in Forgetting Sarah Marshall, shocking his mother so much that she promptly broke down in tears. “Poor Mum,” he sighs. “I really misjudged that. I thought we had this connection. I thought she would find the surprise of it funny. But she really did not.”

The trouble, he explains, is he has grown too old for tomfoolery. He had been playing the same role for so long that he had started boring himself. He feared that if he kept going much longer, he would bore everyone else. So he is branching out, moving on, and leaving the film roles playing catch-up.

It appears the change has been as much physical as mental. Segel used to be a gangly slob. Now it looks as though he’s working out. “Well, yeah,” he admits. “When I hit 30, I started to have that instinct. I started to take care of myself better. New phase, I guess. Stuff starts to hurt in ways it didn’t before.”

He used to be a smoker; I’m assuming he isn’t any more. “Erm, no,” he says, wincing. “I still smoke occasionally. I tend to have one with my morning coffee and then occasionally at night. I’d love to quit, because they’re turning on me. But I started exercising. And I eat more healthily, no more midnight pizzas. It’s no great mystery. Eat well and exercise.”

We may need to wait for his next movie to see exactly what he can do. In the forthcoming drama The End of the Tour, he plays the late David Foster Wallace, the brilliant, brittle author of Infinite Jest and A Supposedly Fun Thing I’ll Never Do Again. The film is based on a series of interviews the writer gave to journalist David Lipsky as the two men ranged back and forth across the US, discussing literature and music, depression and fame. Segel explains that there are not many jokes, which is entirely as it should be. Still, he says that he connected to the script; the whole thing spoke to him.

“Foster Wallace talks a lot about the question of where we take our values from,” he says. “If we take it from external things – success, money, power, or beauty – you’re going to end up empty. And having been in the business for a long time and had every kind of success and failure, I had come out of that journey and realised that its value system doesn’t match with my ethics and values.” He reaches for his coffee. “Look, you can just do this job for the money. But I’ve made quite a bit of money over the past 15 years and it’s lovely, but it’s not enough. I don’t live an extravagant lifestyle. I’m a single guy. I don’t actually need anything. So there’s got to be a different reason why I’m getting up for work.”

I’m still thinking about Sex Tape. It contains a montage in which Segel and Diaz’s characters make love in a variety of outlandish locations; everywhere from the car park to the fiction aisle at the local library. Enough with the talk of ethics and values. Where’s the weirdest place that Segel has ever had sex?

The actor looks blank. He can’t think; he doesn’t know. He insists that his life is pretty boring.

How about we invent a location, to make him look more daring? How about the rose garden? “OK, put that,” says Segel, dissolving with mirth. “The weirdest place I ever had sex was the rose garden at the White House.”

Segel credits his old mentor Judd Apatow with expanding his range and showing what was possible. He explains that he was never going to be obvious matinee idol material. He had to diversify. “I’ll let you in on a secret,” he says. “I act and I write. I produce and write songs. But it’s not like I’m naturally good at any of those things, it’s just that I’ve been prepared to sit in the learning curve. In the end, we’re all just guessing.”

His greatest film, on balance, remains the Muppets reboot, which was heartfelt and supple and a genuine labour of love. Segel developed the project, co-wrote the script and took the starring role as the grownup fan, intent on rescuing his childhood idols. I’m guessing it is the movie that best encapsulates the values he has been discussing.

“Absolutely,” he says. “It’s the ultimate expression. It’s very easy to look at The Muppets as pure entertainment. But even as a child, it taught me so many lessons. One in particular is that the Muppets never try to defeat their villains. The Muppets are always trying to show the villains the error of their ways and get them to come along with them. We can all learn a lot from the Muppets.”

He has collected puppets his entire life. His Los Angeles home is full of glass-eyed, furry animals. “I have the ones I started out with as a kid and used to make films with. But now I have new ones too. When I did The Muppets there was a Jason Segel Muppet, which I eventually got to keep. That’s really the height of arrogance. To have a puppet of yourself in your own house.”

He gulps his coffee and weighs up the implications. “Thank God there’s a context,” he says. “Otherwise I’d be an unemployed man with a house of full of puppets. That would be terrifying.”

The man collects his puppets out of a childhood enthusiasm that has extended into adulthood. Might it be an insurance policy too? Segel saves the puppets and maybe one day they’ll save him in turn; riding to the rescue in his hour of greatest need, or if his new guise starts to chafe. They sit on the shelves of his home, tucked away in readiness, their numbers growing by the year. Just in case.

Sex Tape is released in the UK on 3 September

• This article was amended on 28 August. The original cited Segel’s purported earnings as $80m. This has been corrected.