Today, the Academy of Motion Picture Arts and Sciences nominated Jennifer Connelly for best supporting actress for her role in A Beautiful Mind. But wasn't she the main actress in that movie? How does the Academy decide who is in a "leading role" for the Oscars and who is in a "supporting role"?
It's up to the voting members of the Academy. No rule determines the category for which an actor can be nominated. Every actor in every role in every movie that was released in 2001 was eligible for a nomination for either the leading or supporting award.
How does the voting work? The Academy uses a preferential voting system—members rank up to five preferred nominees in descending order. Click here for a detailed explanation of the system used by the Academy, and click here for a "Chatterbox" that discusses the system.
The 1,315 voting members in the Academy's actor's branch determine the nominees for best actor, best actress, best supporting actor, and best supporting actress. Actors cannot be nominated for more than one award. If an actress gets enough votes to qualify in both the supporting and leading categories, she is nominated in the category for which she got more votes. Once the nominees for each category are decided, the full Academy votes for the Oscar winners, using a more conventional plurality system (each member gets one vote per category, and the nominee with the most votes wins).
The voters who select the nominees can be influenced by the publicity campaigns orchestrated by movie studios to promote their pictures and stars. A Beautiful Mind's ads in Variety and the Hollywood Reporter promoted Jennifer Connelly for best supporting actress, not best actress. That may be because Connelly believed she had a better shot at winning a supporting actress nomination than a best actress nod. According to a Los Angeles Times report from last year, both Samuel L. Jackson (for Pulp Fiction) and William H. Macy (for Fargo) asked to be promoted as supporting actors for what were arguably leading roles in order to give them better shots at winning. (Studios may also push an actor for the supporting category because they don't want voters to divide their votes for one performance between both categories—causing the actor to not get nominated for either one.)