One word - FUN. Here are a few scenes from Behind-the-Scene. Jonah wore that black-rimmed eyeglasses. It makes me want to replace him with Jamie Bell. Sorry Jonah, I still prefer Channing pairing with Jamie.
I also recommend the commentary. Channing said he had never been so happy about his [dick] tips being kissed by a dude. The context is that he and Jonah made a bet about first-week boxoffice number. Jonah said if Channing won, he would kiss his tips through his trousers, and Chan won.