Sometimes you feel like a nut; sometimes you don't. So says Gary Oldman, who decided to play oversexed Puritan Rev. Arthur Dimmesdale in The Scarlet Letter because ''it meant I didn't have to go in and play another psychopathic killer.'' After playing a range of schizy roles, from Sid Vicious to Dracula, ''people don't see me as a comedian or a romantic lead,'' says the actor. ''I don't have the dark good looks of Hugh Grant or Daniel Day-Lewis.'' So what convinced Letter director Roland Joffe that Oldman would be right for the movie? The actor credits the filmmaker's wife, Susie. ''Roland called the part uncastable,'' says Oldman, ''because the minister is tortured, but he had to have a sweetness.'' So she showed her husband 1990's State of Grace, in which Oldman plays a member of the Irish Mob. The deciding moment, he says, ''was a scene where I smiled.'' Talk about sinking your teeth into a role.