According to Mike Newell, director of Harry Potter and the Goblet of Fire, Ralph Fiennes was cast as Voldemort because he has the ability to play the character as a realistic and frightening villain and not as a simple caricature.
According to Newell:
You have to have the actor to do it. And Ralph was the actor to do it, because Ralph doesn't chew the scenery, and he isn't a sort of 19th-century melodrama figure. He's absolutely real and cold and chilling and absolutely dedicated to doing bad. And as soon as you have that, you can do it.
Newell and Fiennes had to ask some simply questions about Voldemort's motivation:
What does he want? Why does he want to come back? What sort of a person is he that he wants to come back? What sort of a person is it who wants to impose a reign of evil? Does somebody who wants to impose a reign of evil actually see that it's evil? Or is it in fact for them a kind of good? So there was a lot of that stuff, which is the sort of thing that you discuss with an actor when he's getting to really concentrate on the part. And that was all very, very profitable.
Newell also gives his reasoning as to why he hasn’t yet read Half-Blood Prince.