Jason Segel juggled his schedule as writer and star of Forgetting Sarah Marshall with his duties on the CBS sitcom How I Met Your Mother.

AP / Gus Ruelas

BOSTON

What surely will be the most remarked-upon moment in writer-star Jason Segel’s Forgetting Sarah Marshall is the two-minute scene he plays nude … totally nude … front and rear … near the start of the film while his character is being dumped by his longtime girlfriend.

It’s a scene in this “romantic disaster comedy” that at first is outlandishly funny. But then, the longer it goes on, it becomes strangely unsettling. It’s a feeling Segel discovered firsthand when he invited his mother and 21-year-old sister to a private screening.

“I thought it would be a funny joke not to tell my mother that I had done it,” he recalls during an interview at the Ritz-Carlton Hotel. “So there’s me, my sister on one side and my mother on the other. And within five minutes, out comes my penis. I turned to my sister and she’s laughing hysterically. And I turned to my mother and she is crying. She said, ‘That’s not a funny joke, Jason.’ She excused herself and left the theater for a 5-minute break. Then she came back composed and watched the rest of the movie … and quite liked it.

“I actually didn’t do it for a laugh, though I knew it was funny,” adds the affable Segel, who looks younger — at 28 — and certainly taller — at 6-foot-4 — than he does on screen. “I think that scene is supposed to portray someone at his most vulnerable and most raw.

“The other thing I like about it is that starting the movie off with something you’ve never seen before in a romantic comedy instantly forces you to throw out any preconceptions about what the rest of the movie is going to be like. I think that’s a fun place for an audience to be, not knowing what might happen next.”

He says the nude scene was shot “in two 14-hour days.” He wanted to play it cool by not shutting down the set to all except a few key crew members as is often the case when actors must perform in a delicate scene. “I thought there would be more pressure if I closed the set or made a ‘deal’ of it.

“Then I showed up and there were 300 people there and suddenly I was terrified. So I drank some whisky and that helped a little bit. But I’ll tell you, it’s still just you walking out there naked.”

The sequence had its germination in Segel’s real “naked breakup, when I got dumped while I was naked. And the whole time it was happening, I was trying to listen to what she was saying, but all I was thinking was: This is the funniest thing that has happened to anybody. I knew I would use it in a movie at some point.”

Forgetting Sarah Marshall was produced by Segel’s longtime mentor, Judd Apatow, and directed by first-timer Nicholas Stoller, another Apatow protégé. So it bears the earmarks of Apatow’s own hits — The 40-Year-Old Virgin and Knocked Up — troubled relationships rollicking with outlandish situations, profanity and sexuality. Segel was one of the stars of Apatow’s Emmy-nominated TV show Freaks and Geeks, which is what started their long relationship, and later co-starred in Knocked Up. Now he’s part of Apatow’s talent stable, which includes Stoller, Jonah Hill and Paul Rudd. Segel describes it as being “a group of likeminded collaborators who are happy to support each other.”

When The 40-Year-Old Virgin became a smash hit, Segel recalls Apatow coming to him and saying, “I can get movies made now, so if there ever was a time you wanted to get one of your scripts made, this is it.” So Segel went off to Hawaii and came back with the first draft of Forgetting Sarah Marshall.

Then it went “into this kind of Comedy Factory where we hone the script. And every time we cast somebody, we rewrite the script for them, which I think is part of the reason why these movies turn out the way they do. We sit in a room and talk and sometimes we sit in a room and improv. We have people taking notes and filming it. Then they transcribe it for us and I go home and turn it into script form.”

It sounds like a humbling experience, but Segel says, “It’s gratitude for having such talented people wanting to be involved with you. Somebody asked me if it were an honor to hear such great actors read my material. I said no, the honor was to hear great actors change and improve my material.”

Yet despite all the improvisation on the set, Segel says Forgetting Sarah Marshall is surprisingly close to the original script. “The nude scene,” he adds, “was sort of the cornerstone of the idea.”

Segel has a lot of irons in the fire right now. Besides traveling cross-country to promote Forgetting Sarah Marshall, he’s still in production on his TV show, How I Met Your Mother, takes time off from that when he can to film I Love You, Man with Rudd, is writing a script for the next Muppet movie and is halfway through another script with Stoller called The Five-Year Engagement. “It follows a couple who get enthusiastically engaged in their early 20s, follows them over five years of seeing if they will actually be able to pull it together and get married, through breakups and cross-country moves.”

Segel says sometimes he films scenes for I Love You, Man on the same day he shoots scenes for How I Met Your Mother, which is about a young man struggling to find his true love. I Love You, Man stars Rudd as a “man who gets engaged and realizes he has no best friend to be his best man. So a week before his wedding he sets out to make a best friend.

“I am THE candidate,” Segel adds with a laugh.

It seems that most of his projects revolve around the problems of young couples. “We like to explore relationships,” he says. Yet he is still single.

“I’ve had a lot of hapless dating over the past 10 years,” Segel explains, “and all my relationships end the same way: It’s me getting dumped and some woman wildly relieved to be free.

“It’s not bad. You get a good movie out of it.”

mjanuson@projo.com