OK, so I was one of a lucky few to watch Judd Apatow direct Seth Rogen, Jason Schwartzman and Jonah Hill in a scene for his upcoming movie "Funny People" (starring Adam Sandler). But, uh, I also saw Gene Simmons going for a walk. He's not in the movie, but he was there, in the Los Angeles hills of Runyon Canyon. Mr. Tongue, Mr. Love Rocket strolled on by and we all just sort of stared at him within this very non-KISS-like milieu. It was an odd day. And not because I was hiking up a mountain (a very slight mountain) in comfortable shoes (I live in Los Angeles; hiking and comfortable shoes are not a part of my day-to-day existence), but because it's simply odd watching a movie being made. You feel a little intrusive. You have an idea what the scene is going to really look like, but not entirely. You don't know what happens at the end of the movie. And ... Gene Simmons just might happen to walk by. Cut!

But this isn't about Gene Simmons; this is about the Los Angeles set of "Funny People," starring Sandler, Rogen, Hill, Schwartzman, Eric Bana, Leslie Mann, RZA and newcomer Aubrey Plaza. Marking his third feature as a director, Apatow appears to be moving into the territory of one of his idols, Albert Brooks, with a movie that's going to be a lot more serious than his previous features ("The 40 Year Old Virgin" and "Knocked Up"). The film's story involves an icon of stand-up comedy and movies, star George Simmons (Sandler), who is sorting out his past and, basically, searching for the meaning of life after enduring the perils and fears of cancer. With this, he comes to struggling young comic (and, according to the script and Apatow himself, a not-very-funny comic) Ira Wright (Rogen) to ask him to write material for him. The relationship moves from star worship and awe to a kind of mentorship, creating a different situation for Ira with his two best friends who are more successful than him: Leo (Hill), a truly funny comedian, and Mark (Schwartzman), a guy who landed a popular sitcom (called "Yo Teach").

Which leads to their scene. Walking through the canyon, Ira, perhaps a little too openly, discusses Simmons' life-threatening illness while his friends react. It's interesting to watch their process, and curious to understand the relationship these guys share. And which one is funniest? Well, all three, but it's intriguing to make a movie about an unfunny comedian starring an actual funny actor and funny person in real life. This has to be a bit tough.

Apatow discussed this during a break from shooting. Because Rogen is so hilarious (watch "Observe and Report," everyone), filming mirthless routines was difficult enough for Rogen to change his character somewhat. As Apatow said, "[It was so hard] to make him look like someone who doesn't know what he is doing [that] I even adjusted the movie a little bit so that he's a little better at stand-up because it's hard to fake not being good. And he's so good you don't want to not use it. So I won't know until editing how good a comic his character is. It's hard to resist using his good jokes as opposed to his bad jokes."

But according to Rogen, it wasn't that hard to not be funny. "It has been interesting. There are several phases, I guess, of my character's stand-up. At the beginning I'm not supposed to be great at it, but you are supposed to see potential, you are supposed to understand why someone in Sandler's character would hire him to write jokes for him. That was kind of tricky, writing jokes that actually were good premises, but delivering them in such a way that you don't feel like I would be the right guy to deliver that material. Then as the movie goes along the character is supposed to start finding some comedic voice and start writing material that is more personal to him and a little more poignant and funnier. It is not my life, for one thing, so to write personal material for someone who is not you creates its own challenges -- and then to be very realistic about how funny every joke is and where they would fall into the movie humor-wise, knowing I am supposed to be a little funnier at the end than I am at the beginning. Really trying to categorize everything like, that joke's funny but it is not personal, that joke's personal but it is not funny, that joke is funny and personal. Finding out has really been pretty complicated."

It does sound complicated. And funny. And not funny. After this very small taste, we can't wait to watch.

"Funny People" opens July 31.