Writer Will Reiser, left, and Seth Rogen fictionalize Reiser’s cancer battle in “50/50.”

Seth Rogen isn’t used to making people cry. The 29-year-old star of “Knocked Up” and “Pineapple Express” is a comedic actor by trade, but in his new film, “50/50,” he’s found audiences let the tears flow as much as the laughs.

“It is weird,” Rogen says. “Honestly, it’s not what we expected.”

The other half of the “we” he’s referring to is friend Will Reiser, who wrote “50/50” after he was diagnosed with a rare form of cancer at 24. “It’s mostly grown men who come up to me [after screenings] and say ‘[Expletive] you; you made me cry in front of my wife. How dare you?’” Reiser says. “And then they say, ‘Thank you.’”

Loosely based on Reiser’s own experiences, “50/50,” which opens Friday, tells the story of 27-year-old public radio producer Adam Lerner (Joseph Gordon-Levitt), who is diagnosed with cancer and given a 50 percent chance to live. Rogen plays a variation of himself, Adam’s best friend and co-worker Kyle.

The film was born out of a necessity for Reiser to find levity after his diagnosis. “I didn’t know how to talk about my feelings,” Reiser, 31, says. “All I wanted to do was feel as normal as possible and not make other people feel weird about it. So I’d just joke about it and we would make jokes, and that was how I dealt with it. To someone on the outside, it would come across as insensitive, and it probably was, but that was the only way I knew how to deal with it.”

As a result, the first draft of his script had strong distinctions between the comedic scenes and the more somber subject matter. “We didn’t know at the time if we’d be able to infuse comedy into the scenes as organically as we did,” Rogen says. “I remember the first draft felt like there were serious scenes and funny scenes, and we figured out how to combine them.”

“What I discovered,” Reiser adds, “is you remind people the cancer’s there. Then, you let them forget about it for a few minutes, and then suddenly something happens. You kind of weave it out but never quite let them forget.”

Rogen Keeps it Real
In “50/50,” Seth Rogen plays a character based on himself, which forced the actor to play it as naturally as he could. “I was more than happy to have people say I was doing the same thing in this movie as I do in every movie — which they say all the time anyway — because I wanted it to seem as grounded as possible,” Rogen says. But his character, Kyle, does have a trait past roles have often lacked. “My characters are usually losers, so they’re not confident,” Rogen says. “This guy’s a loser, but for some reason he’s confident.”