I was watching Silverado the other day, and half way through the movie Jeff Goldblum shows up. Now he did an all right job, but it was just weird seeing him with that hat on and it was kind of funny. Afterwards my brother and I spent like an hour trying thinking of other actors who just wouldn't look right in a western (all-time):
Cary Grant (maybe he was in one?)
Charles Grodin (what ever happened to him)
So who else wouldn't fit (that you still would be curious to see)?