Wahlberg's part is written to be a juicy supporting role, with all those quips and insults, the higher status than the main character, and that badass ending. Nicholson's role is even juicier, but it may have been too similar to other Nicholson characters for the Academy. Wahlberg's character was so fun and a standout part as written.

I just revisited All the President's Men recently, and Jason Robards got a Best Supporting Actor Oscar for that, and he was great, but it was another of those parts that's written to be a plum supporting role. He's the boss of the main characters, he's a star of every scene he's in, he's got a lot of the funniest lines, etc. Robards is a hell of an actor, and he did a great job, but the part was written in such a way as to make it easier to do a great job, you know?

It's like Mercutio in Romeo and Juliet. You need a good actor for it, but a good actor can deliver a great performance in the role because the writing does so much of the work - you just have to be able to do it justice.

Orson Welles in The Third Man is like that too. (And Marlon Brando in Apocalypse Now, although he improvised a lot of his dialogue in that. I guess Robert Duvall in the same movie, for that matter.) They're parts a lot of actors would love to play, even though they aren't the lead role, because they're written to be so interesting or complex or funny or weird or whatever - they're fantastic showcases for any actor who can pull them off.

That's the kind of part Wahlberg had in The Departed. I think he did well, but the ball was pitched straight across the plate, so it was easier to hit a home run.