In a recent interview with The Seattle Post-Intelligencer, Penn Badgley actually admitted that he didn't try out for the role of Dan.
The actor said:
"...initially I wanted to play Chace's (Crawford's) role, Nate. [GG Executive Producer Stephanie Savage] said, 'I'm not sure you can play the floppy-haired, nerdy, sensitive guy anymore, but just take a look at this.'
Eventually they found Chace, and once I saw him I was like, 'Nah, I'm not that guy. I definitely can't do that.'"