Dubiously, she defined her "Reader" role as supporting so she wouldn't compete against herself and the strategy paid off with victories at the Golden Globes (where category placement is decided by an eligibility committee that usually buckles to studio demands) and Screen Actors Guild awards (where stars can place themselves wherever they wish).
But Oscar voters can place contenders in whatever category they wish and sometimes they pay no attention to category requests in "For Your Consideration" campaigns. That's what they did with "The Reader"
A performance by an actor or actress in any role shall be eligible for nomination either for the leading role or supporting role categories. ... The determination as to whether a role is a leading or supporting role shall be made individually by members of the branch at the time of balloting.