You can’t really peg Joseph Gordon-Levitt down. Hollywood can’t, I should say. One minute he’s blasting onto the scene in indies like “Mysterious Skin” and “Brick,” the next he’s tackling blockbuster films such as “Inception” and “The Dark Knight Rises.” Perhaps it’s owed to his self-confessed “eclectic” taste in movies, but the 30-year-old actor seeks out that balance.

“I like a variety of movies,” he says. “’50/50′ is obviously very different than ‘Inception,’ but I loved them both. Variety is one of the things that makes it fun.”

In “50/50,” Gordon-Levitt stars as Adam, a young urban professional in the Pacific Northwest suddenly diagnosed with cancer. Gobsmacked by this, as anyone in his position would be, he rambles to the doctor bearing the bad news. “That’s impossible. I don’t drink. I don’t smoke. I recycle.” And so begins the marriage of comedy and drama that plays to the heart of what Gordon-Levitt calls the honesty of the film.

“The truth is even in the most horrible circumstances, funny shit happens,” he says. “And that’s okay to acknowledge that and it’s okay to laugh at it. In general, we as a culture sort of shy around that sort of humor. We don’t know if it’s okay. We don’t know if it’s proper to find something funny in the middle of something so serious.”

And indeed, the mood on the set was full of jovial spirit and laughter. Seth Rogen also stars, and the film is really his and screenwriter Will Reiser’s story. While working together as writers on HBO’s “Da Ali G Show,” Reiser was hit with a similar bucket of cold water to the face when he was diagnosed, out of the blue, with cancer. But he and his friend got through it with healthy doses of humor along the way.

One might think that, with such a responsibility to Rogen and Reiser’s story, especially with them right there on the set every day, Gordon-Levitt would feel a tremendous amount of pressure. But in reality it was just the opposite.

“It didn’t so much add pressure as alleviate pressure,” he says, “because I knew I wasn’t going to be doing anything false or dishonest with them there. That would be my main worry. If I were doing this movie without Will and Seth, if it wasn’t inspired by a true story, the whole time I’d be like, ‘I don’t know. Does this feel honest? Does this feel real? Is this over-sentimental? Should we be allowed to be making jokes?’ But because Seth and Will were there the whole time, I just felt free to take those risks or try things or underplay things and just be honest.”

Still, the actor wasn’t so much trying to replicate Reiser as a person. It’s not as if this is a biopic or anything, but he was very much interested in trying to translate the profound nature of Reiser’s journey through his performance, who he was before the ordeal and who he became afterward.

“The truth is, based on what Will has told me, he was sort of an uptight and neurotic guy before,” Gordon-Levitt says. “Now the Will I know is such an open-hearted and gentle and sweet dude. Clearly he’s really come a long way, and that was the story that I wanted to tell in ’50/50.’ Not just that I wanted, that IS the story of ’50/50.'”

The film was shot in Rogen and producer Evan Goldberg’s hometown of Vancouver and was the first film they had filmed there, which Gordon-Levitt says felt special. But the actor was particularly receptive to their on-set manner of working.

“They’re really collaborative and improvisational,” he says. “They’ll always shoot the scene as written, but they’ll always start trying other stuff, too. They have a bit of a posse of comedy writers that are producers on the movie and there’s just always lots of spontaneous creativity going on. It’s not just like a recital of what’s written. Not that there’s anything wrong with that, but I think it’s a big part of why the movie feels so real. Often times we were talking off the top of our head and it feels like a real conversation.”

And he’s quick to extend some appreciation to director Jonathan Levine on that, as well.

“That kind of collaborative tone, not every director would be able to handle that,” he says. “[Levine] was, on the one hand, really strong in getting what needed to get done, shot the movie specifically, cut it specifically and he’s great at picking music. He’s a great technical filmmaker. But he also was able to foster that sort of collaborative looseness on the set that I think accounts for a lot of the movie’s honest tone.”

Again, that word, “honest.” You begin to notice while talking to Gordon-Levitt it’s a word that just keeps popping up, the soul of his process. It was clearly a goal for him here — as it is for any number of actors — to get to the root of truth with this particular performance. But for a film like “50/50,” he finds that it’s all the more crucial to find that pulse. Any false note would inherently strip the material of its essence.

It is one of the actor’s finest performances, no question. And he may well pop up consistently along the awards circuit this year. He’s already been tapped for a (rather late) Breakthrough Actor honor by the Hollywood Film Awards. Perhaps that will be the start of a rewarding road for him this season.

“50/50” opens in theaters nationwide Friday, September 30.