Sean Penn was doing a great DeNiro copy in that movie, IIRC, Mystic River.
Does that count? I assume it was a deliberate parody.
Jack Nicholson has made a career out of impersonating Christian Slater in Heathers.
In The Birthday, Corey Feldman does a 100 minute-long Jerry Lewis impression. It’s even worse than it sounds.
Speaking of Corey Feldman, could his performance in Dream A Little Dream be considered a Michael Jackson imitation?
Watch Christian Slater in “Wolf.” There’s somebody making a living copying him…
OK, I’ll bite: Kurt Russell in Escape From New York cruises on a Clint Eastwood impersonation, although to be fair he hasn’t made a career out of it. And he’s back to doing Disney family comedies now {starts humming “The Circle Of Life”}.
I can’t believe that I’m going to admit to having seen this, but in the new version of The Dukes of Hazzard, the guy who played Billy Prickett (the hometown boy gone professional racer), James Roday, was channeling Ben Stiller so hard that it caught everyone’s attention in the room. We just couldn’t decide if he’d modeled him off of just one performance or a composite.
And what? I like Johnny Knoxville. 
It certainly wasn’t nearly as bad as it could have been.
You know, Christian Slater sometimes emulates Jack Nicholson.
And it’s not an imitation, per se, really, but Dennis Farina in Paparazzi (forgive my spelling) is totally channelling Peter Falk’s Columbo. The character was written so much that way that I was wondering why they just didn’t hire Falk, as much as I love Dennis Farina.
Was he trying to do a Woody Allen imitation, or was he trying to do a Larry David imitation?
I mean, George Costanza was supposed to represent the show’s creator, Larry David, a guy who has a lot of Woody-esque mannerisms and neuroses in real life.
My hunch is, the first season, Jason Alexander was trying to be Larry David, but gradually let George become a real, separate character in his own right.
Hank Azaria’s pretty much made a living out of this. So has Harry Shearer.
After all, on The Simpsons, Chief Wiggum is Edward G. Robinson, Lou (the cop) is Sylvester Stallone, Moe is a “bad imitation of Al Pacino” (according to the IMDB trivia page for the show), Harry Shearer’s Seymour Skinner is based on Charles Kuralt, voice-wise. Dan Castellaneta’s Krusty the Clown’s voice is based off of Bob Bell, who played Bozo the Clown.
Pierce Brosnan has taken quite a lot of his style from Cary Grant. It’s particularly noticeable in Remington Steele.
When I started to see the odd rerun of the Jack Benny show, I was more than a little reminded of Kelsey Grammar.
A few years ago, I went to a concert and the opening act was called Common Rotation. There was something very familiar about the lead singer, but I figured he was just giving off a very strong Elvis Costello vibe. Later it was pointed out to me that it was Adam Busch, who had a recurring role on Buffy the Vampire Slayer.
They’re playing here again in a couple weeks. I’ll see if he’s still channeling Costello.
Bradley Cooper did the same as Zack Lodge in Wedding Crashers and since the stars of the movie were Vince Vaughn and Owen Wilson, I can’t believe it was coincidental.
Watching the earliest episodes, I’m going to have to go with Woody Allen, down to the voice. Alexander probably had more access to Woody Allen’s movies than Larry David, face to face.
"Let’s speak of Lamarr, that Hedy so fair,
Why does she let Joan Bennett wear all her old hair? "
IIRC, Jean-Paul Belmondo in Breathless was channeling Humphrey Bogart – it’s even mentioned as such.
In Poolhall Junkies, Mars Callahan seems to be a sort of Christopher Walken Jr. (right down to the crazy hair). Which is really amusing, because Christopher Walken plays his character’s mentor/backer in the movie 