Seth Rogen has become a major star by playing endearing boneheads. In "Knocked Up," "Superbad," and "Pineapple Express," Rogen invested his dopey characters with enough heart to make them sympathetic without getting sappy.

But in his next movie, "50/50," the lovable lunk Seth Rogen plays is Seth Rogen himself.

The film is written by Rogen's real-life pal Will Reiser, and it is inspired by the true story of how the two friends dealt with Reiser's cancer diagnosis while they were in their early twenties. The movie mixes comedy into the dramatic situation, and Rogen told me in an interview this week that he's happy it doesn't always paint him in the most flattering light.

"I am proud to show how stupid I acted at the time," Rogen said, "if it makes other people feel better about how their friends are acting in a similar situation, or how stupid they acted in a similar situation."

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Rogen said that he first met Will Reiser while working on Sacha Baron Cohen's "Tha Ali G Show" when they were both around 24 years old. He recounted, "We became friends, and then I think it was like six or seven months after we stopped working on the show, he told us he had cancer… and we really did not deal with it well." But even as he and Reiser were reeling from the news, they were developing the idea of turning the situation into a film. Rogen said, "I think, as almost a desperate mechanism to try to draw something good out of it, [we] thought, well, it would be great if we somehow made a movie out of this."

Rogen stated that Kyle, the character he plays in the movie, "is much less sensitive than I was in real life," but that he personally related to Kyle's inability to express his feelings. He said the character "is really affected by what's happening to his friend and he just has no idea how to deal with it, and his mechanism… is [to] act like an idiot and kind of try to use it to benefit himself, instead of just talking about it, which is what he actually wants to do." Rogen said that incapacity to communicate on a serious level was taken straight from his friendship with Reiser: "[We] joked about it a lot when he was sick, but we didn't have that many actual serious conversations about it until after."

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Joseph Gordon-Levitt ("Inception," "(500) Days of Summer") plays Adam, the cancer-stricken character based on Reiser. Rogen said that Gordon-Levitt joined the cast "literally a week before we started filming the movie," and that Gordon-Levitt intentionally didn't spend much time around Reiser so he wouldn't imitate the real person on screen. According to Rogen, "[Joseph] really just tried to interpret it in his own way and do his own version of it, which at times was very close to the real Will."

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Rogen was also impressed by how Gordon-Levitt handled the key scene where Adam buzzes off all his hair with clippers while Kyle watches. Rogen said, "It was scary, especially because we were improvising a lot during that scene -- improvising and head-shaving [are] not two things you often find going hand-in-hand." But he was impressed by how Gordon-Levitt retained the dramatic truth of the scene while still finding humor in the scene: "[It] is a very intense moment and the way he plays it is intense, but at the same time it's funny and you can tell it's a guy who doesn't want to be dealing with how intense it is. So instead he kind of tries to make jokes and approach it from a lighter place."

I asked Rogen if there were any moments in the finished movie that felt too uncomfortably personal to him. He replied, "Honestly, the more personal it feels and the more I feel like we are really showing people what we went through in an honest way, the happier I am." Rogen said that he and Reiser felt that truthfulness was the only way to keep the story from running off the rails. He said, "I think as soon as you try to start doing things for the sake of being dramatic or funny and you are not just trying to tell the story, then you really run [into] a danger, especially with a movie like this."