The Oscars, one wants to believe, exist solely for the purpose of recognizing merit in films. There's a kind of wispy romance in that belief, an idealism that overlooks the hard fact that the Oscars are primarily a marketing tool - a way for studio executives to squeeze more money out of their very costly investments.
Want proof? Take a look at the Oscar nominations and the ways in which studio marketing divisions have manipulated Oscar categories to suit their goals. Why, for example, was Catherine Zeta-Jones maneuvered into the supporting-actress category for Chicago while Richard Gere, who had substantially less screen time than Zeta-Jones, was submitted by the same studio, Miramax Pictures, in the lead-actor category? Could it be that Miramax didn't want to pull votes from its stronger best-actress candidate, Renee Zellweger, and figured that Zeta-Jones had better odds in the less-competitive supporting category?
The Hours, which stars Meryl Streep, Julianne Moore and Nicole Kidman, is another case of Oscar manipulation. The actresses appear in interlinking stories and have roughly equal screen time, yet Paramount pushed Streep and Kidman in the lead category and Moore in the supporting slot - even though Moore has 3 minutes' more screen time than Kidman.
It's all about maximizing Oscar potential. Had Paramount put Moore in the lead category where she belongs, it would have diluted Kidman's chances for nomination.
In the past, actors won Oscars as supporting actor or actress even when they had the largest role (Tatum O'Neal in Paper Moon, Timothy Hutton in Ordinary People, Patty Duke in The Miracle Worker).
Conversely, the winner of a lead Oscar (Patricia Neal in Hud or Anthony Hopkins in "The Silence of the Lambs") sometimes belonged in the supporting category.