I like Jack Black,but his part was poorly written....

I think you're missing the purpose of the joke. The whole idea was mocking the industry for doing the typical "fat people, fart jokes" thing. I thought it was hilarious and perfect for Jack Black. The whole movie is making fun of typical things we see from the movie industry.

I thought his character was under-written too, but the fart joke movie is a poor example of that. That was actually a pitch-perfect parody of low-brow Hollywood comedies, and the best of the trailer gags. As someone else explained, the joke isn't "heehee, farts," it's that this character stars in these movies that are hugely successful by appealing to the absolute lowest common denominator of humor, and the trailer is exaggerated just enough to make it funny, but it's actually a really accurate jab at movies like that.

For a dollar, name three white people. . .

I hate Jack Black, and his part was written for Jack Black. Unfunny.

It's not just about fart. It's about family and f...

I think one huge joke EVERYONE is missing is that the lead-in for the trailer indicates that it's a "Fox Searchlight" picture. Fox Searchlight pictures are traditionally those very high-end "arty" films that beret-wearing folks flock to exclusive theatres to watch, then later retire to a local wine and/or martini bar to hold group discussions about the depth and symbolism of the piece.

I've thought I was the only one who didn't like Jack Black in this movie, I thought the scenes he was in weren't really that funny.

I thought he was the funniest part of the movie. A great addition to the ensemble.