For me it was Dan Aykroyd playing Liberace’s business manager Seymour in Behind the Candleabra. I had no idea it was him until I read a review weeks alter. He was completely immersed in the character and unrecognizable to me.
Any familiar actors have you missed in films you watched and only found out they were there after the fact?
Mel Gibson is a chameleon in ‘The Singing Detective’.
I was surprised by, but eventually recognized, Mike Myers as a British General in Inglourious Basterds. But I didn’t realize until looking at the IMDb afterwards that the actor that played Winston Churchill in the same scene was Rod Taylor.
My SO had to point out Keith Richards from The Rolling Stones in Pirates of the Caribbean: At World’s End as I didn’t recognize him.
It took several viewings of This Is Spinal Tap before I identified Anjelica Huston as the designer who built the Stonehenge set which is odd because I spotted Billy Crystal and Dana Carvey right off the bat (and they were playing mimes).
Cate Blanchett in Hot Fuzz. 
I was in the theater watching Tropic Thunder when I heard someone whisper “It’s Tom Cruise.” I honestly don’t know how long it would have taken for me to figure it out on my own. I guess I probably would have wigged to it before the end of the movie.
Not a movie, but for ages I’ve been saying “I like ze black ghostbuster” from SNL.
Only today did I realize that was Julia Louis-Dreyfus.
The first secretary from Monk is the baseball player with the kid in A League of their Own.
Ed Truck and Hank Hooper are the same guy?!
Dan Aykroyd was also in Indiana Jones and the Temple of Doom, as the guy who shows Indy and Willie to their plane at the beginning. He speaks with a British accent, and says “you WILL be riding on a cargo full of live poultry.”
Hank Azaria in Along Came Polly. I don’t think I was the only person that missed that one.
And Thomas Jefferson and The White Shadow!
Huh. The only thing I knew about this movie going in was that Keith Richards was in it. It was well-known Johnny Depp had based his character on Richards and his mannerisms.
Paul Reubens in Batman Returns. He was the Penguin’s father, who sent baby Penguin floating down the river.
That’s a new one for me. I remember thinking I recognized her voice, but I suppose it never crossed my mind it would be anything more than a minor celebrity in the role. Watching it again, the eyes are obviously Galadriel’s.
Gary Oldman in Hannibal for me, although the someone who told me probably did it through the medium of Wikipedia.
I’m a little embarrassed to admit this, but it took me several years to realize that the blind hermit in Young Frankenstein was Gene Hackman.
Speaking of pirates, Glenn Close in Hook.
Top Gun was on TV. I wasn’t really watching it except for any scenes with planes on it, until my brother said “isn’t that Mad Martigan?”
I recently watched Prisoners, starring Hugh Jackman and Jake Gyllenhaal. I didn’t realize one of the other main characters was played by Melissa Leo until I saw the credits at the end.
