Something I spent a lot of time considering as I filled out my Muriels ballot this year:
If an actor in a role that isn't billed as the lead role (let's call him Heath) completely takes over the film, is he really a supporting actor? Isn't he then an Actor in a Leading Role? Doesn't that inherently go against the whole point?
To me, a supporting actor is one who makes everything and everyone else in the film better, not someone who dominates his or her scenes. This isn't basketball where coming off the bench and taking over is a virtue.
 My girlfriend points out that she said pretty much the same thing 3 days ago at a bar. She's usually right (and when she isn't, it isn't worth pointing out).