‘Forgetting Sarah Marshall’ Star/Writer Jason Segel Has A Terrifying Run-In With A Future Co-Star

'I lived in terror of this guy for a long time,' Segel says of the intimidating ultimate fighter who pops up in the comedy, which opens Friday.

They say that a movie can only be as good as its story. On Friday, millions of theatergoers will follow in the footsteps of those who made “Superbad” and “Knocked Up” big hits, going to see “Forgetting Sarah Marshall” with hopes that a character’s constant run-ins with his ex will yield yet another Judd Apatow comedy classic.

But behind the scenes, one story may have been even better. “Did you ever hear the Claw story?” director Nicholas Stoller asked recently, gearing himself up. “Oh, you’ve got to ask Jason about that one.”

He’s referring to Jason Segel , the fast-rising star of “Marshall” and the hit sitcom “How I Met Your Mother,” who also wrote the film about a downtrodden dumpee who runs across the breaker of his heart (Kristen Bell) and her new boyfriend (Russell Brand) while on a Hawaiian vacation.

“Yeah,” the 28-year-old funnyman sighed when asked about the story. “That was truly the strangest coincidence of all time.”

“Jason spent a couple months in the summer [before we shot the movie] working in a house on the North Shore [of Oahu], where he was writing the first draft of the script,” Stoller remembered. “This ultimate fighter named Claw trained on the beach nearby and wrapped these mattress-pad-like things around a palm tree, and he’d just beat the sh– out of the pads all day. There were these girls walking down the beach, and when girls would walk past [Segel's porch], he would lift his beer up and say, ‘Hi.’ Well, these chicks said, ‘No!’ — and they said it very seriously.”

“While I was writing the film I accidentally flirted with this girl, who turned out to be [dating] the scariest man I have ever met,” Segel confirmed. “He was an ultimate fighter, and he basically threatened to beat the sh– out of me.”

“[Segel] turns and sees Claw, and Claw is like, ‘What the f— are you doing? Are you f—ing with my girl?’ And Jason said, ‘No, sir!’ ” Stoller laughed. “So Jason was terrified for a month after that, because he was afraid he’d run into Claw on the North Shore. It’s such a small community.”

“I lived in terror of this guy for a long time. … So when that happened, I wrote a scene about it, because I thought it would be a funny thing to happen in the movie,” the writer/actor recalled.

Added Stoller: “This is island culture. People get bored, and people who fight the next day are best friends. So we based the fight in the movie on that same beach, and I cast a guy who was a surfer named Kala Alexander.”

“When I showed up to do the movie, Nick said, ‘We cast a really scary guy to intimidate you in the fight scene,’ ” Segel chuckled. “And I look at the head shot, and it’s that dude! It was horrifying!”

“I didn’t realize [when I cast him]. … Jason turned really white, and I said, ‘What is it?’ ” Stoller said. “And he just goes, ‘Claw, Kala. Kala, Claw.’ He had realized they were the same guy.

“Jason got f—ing terrified [that] he could remember him from almost beating the sh– out of him before,” the director recalled. “And I said, ‘Look, Kala is a working actor. He doesn’t want to beat the sh– out of you. He just wants to do the movie. He doesn’t care. You’re not currently hitting on his girlfriend!’ ”

When theatergoers watch the “Forgetting Sarah Marshall” scene in which Segel’s character, Peter Bretter, goes out on a date with Rachel (Mila Kunis) and ends up getting beaten up by her ex-boyfriend, they might be impressed with the star’s acting. But the guys insist there was no acting involved on that day.

“We shot the scene, and the whole time Jason is terrified,” Stoller laughed.

“I never mentioned it, and he never mentioned it,” Segel recalled. “But I know that we both knew.”

Check out everything we’ve got on “Forgetting Sarah Marshall.”

For breaking news, celebrity columns, humor and more — updated around the clock — visit MTVMoviesBlog.com.