Why is Rachel Weisz considered to be a supporting role in The Constant Gardener and not the leading actress category?
I know she's won the Golden Globe and the SAG award & has been nominated for the Oscar in that category, so the film industry has no problem with it. But with the amount of screen time her character has, I don't get it.
How is supporting vs lead determined?

Actually, I thought Reese Witherspoon's role in "Walk the Line" was clearly a supporting role, obviously I was wrong. But that was the impression after seeing that film.

I don't have as much issues with Weisz in the supporting actress category because of her role in the film, and Ralph Fiennes' role as a lead.

Usually actors (male or female) are placed in Oscar categories where they think they might have a better chance to win and not based on screentime.

Case and point :
Anthony Hopkins in Silence of the Lambs - supporting role not lead
Marlon Brando in The Godfather - supporting role not lead
Al Pacino in The Godfather - leading role not supporting

and I am sure that there are plenty of others

Ethan Hawk Supporting role and Denzel Washinton lead role for Training Day WTF?

There's no rhyme or reason for lead vs. supporting for awards. Jamie Foxx had more screen time than Tom Cruise in Collateral (he was in like 98% of it) AND played the character that drove the film, but was up for Best Supporting Actor. Maybe that had something to do with also getting the nomination for Ray, but I think that's a major example of how they do things.

Also, Judi Dench got Best Supporting Actress for like 7 minutes worth in Shakespeare In Love, and Nicole Kidman didn't have a huge role when she won Best Actress for The Hours.

i can see why Weiz was up for supporting as she's only in the movie for about 30 minutes or so.
another controversial one was for Catherine Zeta Jones in a supporting role in chicago. (she even got top billing)

The studios submitting the nominees decide which category to place them into competition for. In general, they choose the category which would give them the best chance which is why you'll see things like Ethan Hawke or Jamie Foxx up for best supporting, when obviously in those films they weren't really supporting roles (Collateral, Trainging Day).

Whoever can resist their gag reflex more.

Seriously, it's probably some mathamatical thing. Like the chances of them winning in that category over another, and then you factor in if a studio has multiple nominees...
I wouldn't take too much stock into the whole screentime thing. I think it's a myth.

Yeah, that was pretty bogus in my mind too. I don't mind Denzel being consider a lead role, but the movie was about Hawke. It should have been two leading roles. The studio was obviously trying to garner a nod for Hawke, which sucks that they let such games be played.

As for Weisz, I can see supporting role since she's only in the film up to a point. I don't know her total screen time, but it's not like she's always there.

Anthony Hopkins won for lead actor for SOTL. Just sayin'.

He sure did. (That was the year Jack Palance won best supporting actor for City Slickers)

Yeah, no one wanted to see Anthony Hopkins doing push-ups on the stage that year.

i think inri222 was saying that Hopkins and Brando were playing supporting characters and got lead Oscars while Pacino played a lead character and got a supporting Oscar.

The Golden Globe people wouldn't let Jake Gyllenhaal into the supporting actor category because his role was too big but I think it is a supporting role compared to Heath Ledger's character.

When there is ambiguity about what category --supporting or lead, that a performer ought to be nominated in, the issue is decided by what category the performer campaigns to be nominated in.

[QUOTE=mongoco]i think inri222 was saying that Hopkins and Brando were playing supporting characters and got lead Oscars while Pacino played a lead character and got a supporting Oscar.
QUOTE]

Well you got it partly right, the only thing is that Pacino was nominated but did not win.