According to the DVD documentary, Jim Carrey was originally offered the Jeff Daniels part because it had more dialogue and depth. He chose the other role because he felt it had better jokes.

Chris Elliott (Get a Life, Something About Mary) was signed, sealed and delivered for the Jeff Daniels role, but he had to bow out due to contractural commitments to Frito Lay (which required him to film a series of commercials at the same time Dumb and Dumber was shooting).