Often times actors will be cast in totally different roles than the one they auditioned for. There seems to be a recurring thing in Hollywood right now where people go in gunning for the hero role, and wind up being cast as the antagonist instead. The casting switcheroo in Dexter wasn't like that, but it was still pretty odd. Julie Benz was a series regular in the early seasons of Dexter as Rita Bennett, that sweet, naïve and totally doomed single mother that the main character shacks up with in the hope of grounding himself in a more normal life. Which obviously all goes to pot when she gets targeted and murdered for being involved with Dexter. Before she got the Rita part, though, Julie actually auditioned to play Debra. You know, the foul-mouthed adopted sister of Dexter. Which is a complete 180 from her actual role, both in terms of relationship with the main character, and with how often Benz would have had to say the f-word during her time on the series.
Tom Baker is the Comics Editor at WhatCulture! He's heard all the Doctor Who jokes, but not many about Randall and Hopkirk. He also blogs at http://communibearsilostate.wordpress.com/