Dallas has television commitments that prevent him from shooting the movie.
Interestingly enough, it was Zac who was offered the part for THOR but had to back out when CHUCK was picked up for more episodes. So now the two have replaced each other in the series.
Though, which one of them will play Fandral in AVENGERS 2?
Check out Zac's appearance on Leno below (thanks to chucksarahmedia for the upload).