Johnny Depp doesn't get the respect he deserves. Maybe it's because he started out in television, or because he's pretty enough to offend a certain class of people. He was brilliant as the young, pathetic Ed Wood. His performance in Donnie Brasco makes you think he could play the old, decrepit Ed Wood too. There's real depth and maturity in his Donnie.
Al Pacino's in the movie too, and he is terrific. His Lefty was once a tough-guy, and you can see that in the character, but you also see that Lefty doesn't have the drive or ambition that Donnie does. He's gone about as far as he can in the organization, and at his age is more or less content to get by, play by the rules, and make a comfortable living. But he's lonely, and so he befriends Donnie. He begins to live vicariously through the younger man. He sees in Donnie the potential that he himself may once have had, but squandered. All of this comes through in a expertly-nuanced performance by Pacino.
The two actors play off each other well. Both are trapped in the game they're playing, and it's fascinating to watch how they deal with it, each in his own way.