A lot of this has to do with the way the part is written. Robert Downey Jr's character satirizes Method actors, actors taking themselves too seriously and the very present racial stereotyping still existent in film. Stiller, Justin Theroux and Etan Cohen (not Ethan Coen) created these intentions within the script. But Downey Jr. brought it to life in a way that exemplifies the points that the writers are trying to make with his character. It also works on another level because Robert Downey Jr. is essentially making fun of himself. He basically is a Method actor (he stayed in character during the shoot). On top of it all, let's not forget that he is playing an actor playing an actor playing a character. The actor he plays is Australian Method Actor Kirk Lazarus. Someone whose Method acting techniques have prevented him from knowing who Kirk really is. He has to play this as well. When Alpa Chino (genius) and Speedman point out to him the absurdities of what he is doing, a hint, a slight hint of Kirk shows through. It's barely visible, the camera does not point it out but it is there. And on top of all of this he has to be funny. And holy shit he is funny. It goes without saying that he steals this movie. His speech, his facial expressions, his mannerisms are all spot on and his line deliveries are some of the best I've seen in years.
So in a year where Heath Ledger will undoubtedly win the Oscar for Best Supporting Actor (even though I'm already rooting for Downey, but I obviously won't be upset when Heath gets it) can it really hurt to just nominate Robert Downey Jr.? If his performance is not enough to do it can't his comeback push him into a nomination? In a year when his performance in Tropic Thunder can garner some of the best performance reviews of any this year, I don't think it is asking much for him to get in. The notices and praise he has recieved has been equal to any performance this year.
Between the universal acclaim for the performance, the positive critical reception of the film, his comeback, how impossible of a performance it is to pull off, how many layers the performance has to work on and it being the funniest performance of the year, how is this not a lock? I agree that a comedic performance is hard to nominate. Not many should be. There are so many comedic performances that I think are hilarious that should never in a million years be nominated for an Oscar. But this one should. Some people seem to think Tropic Thunder is just one of those movies. It may come off that way. Although it's not perfect people seem to forget that the reviews it got were better than the vast majority of films that came out this year. Peopel who understand the film industry get what the film is doing with that and the other levels that it works on beyond the basic comedic level. "Tropic Thunder" does something more unique than most comedies. And Robert Downey Jr.'s performance is a big part of that. Therefore, just nominate him. The Oscars are a joke anyways so at least nominate something that deserves it and shows you are going out on a limb to recognize unconventional performances.
Here's a link that can explain this better than I can.http://www.cinemablend.com/new/Editorial-Robert-Downey-Jr-Deserves-An-Oscar-For-Tropic-Thunder-9797.html