What qualifies as a lead versus a supporting actor?

I don't know the answer to that question, but I have a feeling it may be about the egos of the actors involved. It sure doesn't have anything to do with the number of lines or screen time. William H. Macy was on-screen 15 minutes longer than Frances McDormand in "Fargo," yet she won an Oscar for lead actress and he was nominated for supporting. Ethan Hawke had the most lines and screen time in "Training Day," yet Hawke lost a supporting actor Oscar and Denzel Washington won in the lead category. Nicole Kidman had the fewest lines and screen time among herself, Julianne Moore and Meryl Streep in "The Hours." But Ms. Moore voluntarily asked to be considered a supporting acrtress and was nominated for an Oscar. Ms. Streep decided to take on Ms. Kidman in the lead category and wasn't nominated. I'm betting if Leonardo DiCaprio had told Oscar voters to put him in the supporting category, he would have won the Oscar for "The Departed." Instead, some voters supported him for lead, others for supporting. He wound up with a lead nomination that year for "Blood Diamond," which was nothing more than a consolation prize. Instead Mark Wahlberg, of all people, got "The Departed" supporting nomination.

I bring this up because of the upcoming film "Frost/Nixon." Right now Frank Langella as former President Nixon, seems a lock for leading actor nomination. I would have thought Michael Sheen, as TV commentator David Frost, would try for a supporting nomination even though he has more screen time and lines than Langella. After all, Sheen had more screen time and lines than Helen Mirren in "The Queen" and he tried to get a supporting nomination from that effort. But now I understand Sheen wants to be considered a lead actor. Technically, he is, but, realistically, I don't think he stands a chance of winning anything against Langella. Perhaps Sheen thinks the shadow of Heath Ledger is just too difficult to overcome in the supporting category this year.