Best Actor and Best Supporting actor nominations are based on what you're submitted as.
George Clooney as Best Supporting Actor for Syriana? It was an ensemble with Clooney as 1A, Damon as 1B and Wright as 2, but it was mostly Clooney's movie. Nicole Kidman wasn't on the screen that much in The Hours for her Best Actress. Hell, Jamie Foxx was a Supporting Actor nominee for Collateral, and he was the driving character and in it for like 99% of the run time. Ethan Hawke getting a Best Supporting nomination for Training Day? Judi Dench won Best Supporting for five minutes of screen time in Shakespeare in Love.
The actors get submitted based on what they think they're more likely to win.