Chris's family have been working-class actors; and were his grandmother alive, she'd be celebrating her 80th anniversary starting in show business. Chris has said the unglamorous side of the business grounds his perspective; and that shows in his approach to interviews, which is fairly workaday. He and Zach have fun in them, but Zach looks to be the impressionable one to me.
Zach doesn't seem impressionable at all. He gives off the vibe of someone who doesn't really care what people think and would never do anything just to please other people. (I just remembered a story that J.J. told in an interview, during the shooting of the movie, Zach would rest after all the running he did in the action scenes, and J.J. would tell him "When we were making Mission Impossible, Tom Cruise would exercise in between takes!" Zach just answered: "I'm not Tom Cruise".

) He seems very serious about his acting but doesn't seem to care much about the whole celebrity thing. He is nice to fans and ca be in very good mood with some of the interviewers, but he often seems bored with photographers taking his pictures at public events, and I love the moments during some of the interviews when he gets asked a stupid question and glares at the interviewers as if he'd rip his head off

before giving a serious answer that manages to ignore the stupid angle of the question.
He seems like such an offbeat character, not a typical Hollywood person at all. He's founded a production company with some of his friends, called Before the Door - apparently they're currently developing a movie Zach will produce and star in, about the economic crisis. He says he'll play a financial analyst, and that it's a bit like Glengarry Glen Ross. Did you see his Youtube channel (or rather, his production company's YT channel)?
http://www.youtube.com/user/beforethedoor The video he posted (and wrote a note for), a short movie made by his friends
http://www.youtube.com/watch?v=bUdm5oOnt7o Totally bizarre stuff!
http://www.youtube.com/watch?v=bUdm5oOnt7o