I don't think the deciding factor in deciding whether an actor should be lead or supporting has anything to do with screen time or whether someone else in the movie has a bigger role or anything like that. It's more just about whether the role is substantial enough to be competitive in the lead category. If it is, it should go there; if it isn't, then it should go under supporting.
Jackie Earle Haley, for example, should be supporting.
pixote