Some more tidbits from Michael Cera on Jean and the making of Youth in Revolt:

CP: There’s also this idea of authority in the book and the film, which I hadn’t really read elsewhere: All adults are horrible and that’s really all you have to look forward to — being horrible yourself.

MC: I think that’s what’s really interesting about the casting of the movie. [Nick’s] parents are these really great character actors: Steve Buscemi and Jean Smart. They both played their characters really gross, in a really great way. I think that tells you a lot about Nick and why it means so much for him to meet this girl. You see the people he’s been stuck with his whole life and he can’t stand them. It’s really important for him to hang onto this person and it’s representative of what his future could be or not be.

CP: When you read the book, how did you visualize those adult parts?

MC: I visualized Jean Smart actually. I didn’t know where I knew her from but I knew her face.

CP: You weren’t watching Designing Women all the time?

MC: No, I never got into Designing Women but people tell me she’s amazing on that show. She’s an amazing actress. She had a broken leg when we were shooting.

CP: No way!

MC: Yeah, she had a broken her leg just a few weeks before and she took her cast off while we were shooting and had to walk around in a few scenes and it was excruciating for her.

CP: You don’t see it at all.

MC: Yeah, I felt very bad for her. But she was who I pictured when I was reading the book.

(Source)