Breaking up is never fun. Whether it's just a summer romance, a courtship of many years or even a marriage dissolving, it's hard to find a way to smile. Jason Segal found that way by writing a comedy script. The star of TV's How I Met Your Mother and films like Knocked Up, Segal was a regular in Judd Apatow's short-lived but critically acclaimed series Freaks and Geeks and Undeclared. Now that Apatow is a Hollywood mogul, he's producing all his buddies' movies.

The story of Forgetting Sarah Marshall has the title character (Kristen Bell), a famous TV actress, dump her boyfriend of five years, Peter Bretter (Segal). To escape his depression, Peter treats himself to a vacation in Hawaii, only to find that Sarah is there with her new boyfriend, rock star Aldous Snow (Russell Brand).

Much of the film is autobiographical, from Segal's own Hawaiian vacations, to the initial break-up scene where Sarah catches Peter fresh out of the shower. The R-rated film goes all out with Peter's vulnerable state, setting the film's outrageous tone.

"I had a naked break-up," admitted Segal. "It was basically exactly as it was in the movie. I thought this woman was coming over to have sex with me, because she was my girlfriend and girlfriends and boyfriends have sex. So I was naked when she arrived and she walked through the door and I literally did, 'Hey, baby.' She looked me in my face and said, 'We need to talk.' Instantly I knew it was happening, and the problem, the sickness is that I knew that I should be experiencing this viscerally, like this is two human beings having an actual moment. All I kept thinking was, 'This is the funniest thing that has ever happened to anybody.' Honest to God, like, 'I can't wait till she walks out of this room so that I can start laughing and call all my friends.'"

Sarah actually asks Peter to put some clothes on, but he refuses to make it easier on her. "You know, that was a very big day for Jason," said Bell. "It was one of the last things we shot and we were all really close as a crew and as a cast so we just wanted to make him feel comfortable. We tried to make him feel as comfortable as possible, but then there was a point where he got comfortable enough that we were so clearly in coverage and he was still naked. I was sort of like, ‘What’s that frame on? We’re here, right? And he’s still here.' There was some sort of nylon sock involved which I’m going to tell you just draws more attention to the area than if it didn’t have a nylon sock on it. So, I just hope he was comfortable. I think he did a great job and I’m kind of proud of him. I’m lucky he’s taller than me and my eyes stayed to the sky."

In Sarah's defense, Peter is a bit of a couch potato. Lying around the apartment eating cereal in sweatpants is not the best way to hang onto a glamorous celebrity. Bell, however, has been guilty of that herself, despite her own glamour.

"I’ve been the couch potato," she said. "I tend to work very hard when I’m working and then in my off time I barely leave the house. I’m definitely one or the other. I will either have three jobs at one time like I do right now or I will just be completely stuck in bed all day, every day, watching reruns of Jeff Corwin and just happy as a clam."

Sarah is no saint either. She relegates Peter to the sidelines, where he is often photographed holding her purse in the background of a red carpet photo. The real life Bell is more sensitive than that. "I’ve had a couple different people in my life hold my purse so I would never ask anyone – friend, boyfriend or anyone – to do it too many times. I think it really does strip you of your dignity a little bit. This is embarrassing.”

Either way, what's done is done and there's no getting Sarah back. So Peter first attempts a series of one night stands to help him forget about his ex. These women turn out to be even crazier than she was.

"Sometimes I think you go a little bit nuts after you've been in a long term relationship," said Segal. "Peter has been with Sarah Marshall for five and a half years, so I think kind of the first thing you do to cleanse yourself of all the love feelings is to replace them with just regret and dirty feelings. The whole movie is kind of an amalgam of strange relationships and breakups and weird encounters with women. Some of them were more realistic than others and some of them came from things in [director] Nick [Stoler]'s life or things in Judd [Apatow]'s life. We're a pretty collaborative group, so it's sort of like once you get riffing on the idea that you're going to do a sex montage, you'd be surprised how many stories come out in a room of comedians."

Even without his ex-girlfriend showing up, Hawaii does not seem like the best vacation spot for a newly single man. He's sure to be reminded of romance by all the honeymooning and otherwise romantic couples frolicking around.

"I'm gonna be honest with you, I'm pretty big on the self abuse," said Segal. "I think it leads to the best comedy, which is why I set the movie there. I think misery in paradise is just a funny pairing. I remember early on in the writing process, picture me crying hysterically in the front of the most beautiful sunsets you've ever seen. All those ideas started to lend itself to Hawaii being the place."

Of course, seeing your ex with her new rock star boyfriend there is just bad luck. Playing Peter's replacement, Aldous Snow, is British comedian Russell Brand. Famous for his womanizing antics in the U.K., Brand made a splash at his Hollywood film audition. Snow was originally to be a Hugh Grant type Brit, a stuffy author. Brand's attempts to sex himself up led him away from the literary arts.

"Authors can be quite strapping characters like Hemingway," explained Brand. "He was certainly no wallflower. So within literature, there are people that are quite decadent and hedonistic, aren't there? Henry Miller. So I just thought all right, I'll play him like he's a Henry Miller kind of sexy author. Then they thought, 'God, it'd be easier to just cast him as a rock star rather than trying to justify a man who writes books for a living having that amount of eyeliner on.'"

That might also explain to American audiences how Snow steals Sarah away from Peter. In real life, the comedian Brand does so well with the British ladies that the tabloids make him a regular beat for their reporters. Now he has a whole new smorgasbord of American babes.

"I think they're very beautiful," commented Brand. "I mean, I love English women. My mother's an English woman for heaven's sake but yeah, I like American woman. I can see what Jimi Hendrix was on about when he said, 'American Woman, wooooah!' I think people [in the states] are very garrulous and available and I enjoy that. I live, of course, and am from a culture where people are deeply, deeply repressed and constantly embarrassed by their own genitals. I celebrate mine. I try to let them lead. Yes, I'm something of an anomaly in English culture."

With such evocative language, and wardrobe reminiscent of Johnny Depp in the Pirates of the Caribbean movies, it's easy to buy into Brand's hype. His leading lady, however, suggests that this is all an act.

"The thing about Russell is when he’s on, he has the vocabulary of an alien and he is hysterical," said Bell. "But when he’s off and more just one on one, he is really kind and he is much softer. We had a lot of sex scenes together and we didn’t really know each other that well but he was so comfortable to work with and quite protective actually. That’s why I’m planning on outing and destroying his street cred because when we would do it, he would always make sure that I was tucked in like a burrito with the sheet when the cameras weren’t rolling."

Cavorting with a rock star in front of her distraught ex seems a bit coldhearted, even if it was just an unfortunate coincidence that Peter ended up on the same island, at the same hotel and in the room next door. Still, the title character of Forgetting Sarah Marshall is not just the villain.

"There were always elements of being sympathetic," she said. "I think we have the minimum amount. The more you can get in there, the better, though I completely trust these filmmakers and their editing process and think that it’s exactly where it should be right now. It was definitely a balance of wanting to make her likeable but also there’s so much comedy in the ditsy, bitchy parts of her, but also accurately portraying that she did have her reasons for breaking this up. It wasn’t what was fulfilling her any more. It wasn’t out of being mean. It was just about it doesn’t appeal to someone anymore to be with someone who has completely locked themselves in the apartment and survives solely on Fruit Loops. She was looking for something different and it was an unfortunate thing to admit, but I think that a lot of the speeches let you in on her thought process and didn’t demonize her so much."

Forgetting Sarah Marshall opens April 18