First, Ian Somerhalder previewed his character in The Vampire Diaries.
Now, it's Paul Wesley's turn.
In an interview with iesb.net, the actor goes into detail about Stefan Salvatore. Here are a few excerpts:
On whether Stefan is all good: He's a vampire. He is constantly struggling with the dark side. He's constantly eager, and he has a need to feed on human blood. It's always there. It's not easy for him not to do it. I'm making Stefan have anger and a temper, but also passion, love and tenderness toward Elena. He's vicious and violent. He loves her and he'll die for her, but he's a violent creature.
On differences with his brother: Stefan hates being a vampire. He doesn't want to be a vampire. He believes it's immoral. He has a conflict. He does not want to take human life. He just wants to experience life as a normal human being, whereas his brother is completely basking in this. He loves vampirism. He's enjoying it. He's more gluttonous, in that regard.

The Vampire Diaries premieres on September 10 on The CW.