When I meet him, in the cavernous LA warehouse that houses the sets for the enormously popular Fox sitcom How I Met Your Mother, Jason Segel is tired. It's understandable: he spent most of last night reshooting scenes from one of his three forthcoming films. Now he must get into the character of Marshall Eriksen, whom he's played throughout seven series of the Emmy Award-winning sitcom and invested with the trademark sweetness that can make even the creepiest and neediest characters seem lovable. We take advantage of a break in filming that has left Studio 22 deserted to settle down in a beat-up couch.

He is a huge man, at least 6ft 4in, and exudes a strange mixture of self-consciousness and calm. He doesn't seem to take himself seriously at all, but the 32-year-old has become one of the major figures in Hollywood comedy, not just for his acting but as a writer and producer, too. Along with Paul Rudd, Seth Rogen and Jonah Hill, he's part of the Judd Apatow coterie – the chosen ones whose shtick as endearing, underachieving slackers has created a series of wildly successful films, from Knocked Up to Forgetting Sarah Marshall, which Segel wrote and Apatow produced. A Knocked Up sequel, This is 40, will be out later this year.

Now, however, he has quite a different project at the forefront of his mind. He has written, executive produced and taken the lead role in the first Muppet movie to hit the cinemas in 12 years. The Muppets has already grossed more than £100m in America.

Like many of us, Segel loved the Muppets as a child: "It's your first entrance into comedy that's kind of subversive. The Muppets lead on to Monty Python and Saturday Night Live." But it was only after the success of Forgetting Sarah Marshall that the opportunity to work with Kermit, Miss Piggy, Fozzie Bear and Gonzo presented itself.

One of the standout moments of Forgetting Sarah Marshall was its opening scene, in which Segel's character is taking a shower when his girlfriend dumps him. Segel was unashamed to milk the comedy, revealing a pasty, doughy body in uncompromising full-frontal nudity. The film also included a puppet musical of Dracula which Segel wrote. It was after this that Disney invited him in to chat about his plans. "I was at a point in my career where I had a very neat little moment and people were interested to see what I would do next," he remembers. "The expectation was that I would churn out R-rated comedies and cash in while I had the chance, but it's not the dude that I am. I knew I wanted to make the Muppet movie, so I went to Disney and I blindly pitched. They thought I was joking."

He was not. The movie he's made is about rediscovering the Muppets, and about its joyful mingling of adult and childhood comedy. Segel surprises me when he says that Peter Sellers is his "idol" and he brings a framed photograph of him on to every set – Sellers, after all, was not famed for his niceness, whereas Segel is the epitome of it. But the comedian's guest appearance on The Muppet Show is one of Segel's earliest memories of the programme: "Scooter comes in. Peter Sellers is sitting there. 'Five minutes to curtain, Mr Sellers.' Peter Sellers says: 'What am I supposed to do there?' Scooter says: 'All you have to do is be yourself.' And Sellers launches into an existential crisis: 'Who is Peter Sellers? I don't know myself any more.' This is why The Muppet Show is so brilliant. Parents and kids are watching the best comedian of their generation, and both are enjoying it."

And then there's his love of puppets – I've heard he has a lot. "It gets exaggerated. I started off getting toy store puppets, then nicer ones. The fact I wrote The Muppets makes it less creepy. When you're an unemployed guy with 25 puppets it's creepy."

He's particularly fond of the Oscar-nominated song "Are You a Man or a Muppet?", which was written by Bret McKenzie of Flight of the Conchords fame. It leads me to pose the slightly unconventional question: which one is he? "I don't think they are mutually exclusive," he replies. "The Muppets are a troupe of comedians, a collective – they are not just puppets." When he adds: "The Muppets don't care much about people loving them; they care about taking care of each other," I wonder if he's talking about himself.

Segel grew up in Pacific Palisades, a rich urban area in Los Angeles. "I was a high-school athlete. I was an athlete my whole life until I became an actor, so that was a different lifestyle." His parents expected he would be a lawyer or a doctor, something serious. "That's where the writing came in. The Muppets took five years to get right, although I wrote Sarah Marshall in a month – it just flowed out of me."

Forgetting Sarah Marshall drew much from his own life. The naked break-up scene was "verbatim. It was very close to what happened with my girlfriend of five years, except one part was cut out." As in the film, she called to say she was on her way from the airport. Segel was just out of the shower. "Halfway through this break-up I looked down at my naked body and tiny shrivelled, embarrassed penis and thought: 'I have to put some clothes on.' I go to my closet. Picking the outfit for the second half of the break-up is the hardest outfit you'll ever pick. I was ripping clothes off hangers, throwing them to the ground, until I came out in khaki pants and a button-up blue shirt like a little schoolboy, and I said: 'I'm wearing your favourite outfit.' It was not her favourite outfit. I don't know why that came out of my mouth."

While this was happening he was thinking: this will be in a movie. "I'm keeping track of what's being said and thinking: 'This is the funniest thing that ever happened.' Half of me is devastated and half of me is not in the moment, disconnected from reality. That's the way I get through life. I'm a watcher more than I am engaged."

Next up he stars with Emily Blunt in The Five-year Engagement, and also in the Jay and Mark Duplass comedy Jeff Who Lives at Home, as the "space cadet" brother of Ed Helms (of The Hangover and the US version of The Office). Susan Sarandon plays their mother. "I just say the words written on a page," he says, "but watching her you see she really has a process. That's why these people get nominated for Academy Awards." His head's down and he's rather mumbly by this stage. "I just do the face that matches the words."

He may not be starry, but starry things happen to him. Recently an attractive 22-year-old fan wrote the "Jason Segel Song" and posted it on YouTube. He was touched and invited her to come to a comedy show with him. "It wasn't a date. It was just a nice thing to do. My people were like: 'Can we make sure she signs a non-disclosure.' I had to say: 'Guys, calm down. Let's remember who we are. This is a girl who wrote the coolest song ever. We're going to do something nice and invite her to the show. We don't need a background check.'" You get the impression Segel will never forget who he is no matter how huge he is surely destined to be.

• The Muppets is released on 10 February