He plays Zach, the impossibly perfect new boyfriend of Summer. ''He's very preppy, very charming, the son of a politician, a water-polo player, and he loves comic books,'' says the 21-year-old actor. Seth describes Zach as ''the WASP version of me.''
How you know him You probably don't. But Cassidy did screen-test for Superman during the time The O.C.'s executive producer McG was slated to direct the film. ''I didn't get the part, but he sent me over to the show's producers, and that's how I was cast.''
The O.C. vs. his real high school ''I didn't know too many girls who dressed like Marissa and Summer!'' says Cassidy, who attended LaSalle High School, in Portland, Ore.
O.C. role model ''I was more like Seth than Ryan in terms of feeling like an outsider in high school. I was geeky, and dealt with it the same way he does, by being funny and sort of using comedy as a creative outlet.''
Voted in senior year as ''Most Likely to Be on Broadway, or something like that.''