Hey, It's That Guy!: John Malkovich, better known as Cyrus in Con Air. Also in-universe, everybody recognizes him from various movies they can't name, some of which he was never in.
Octavia Spencer as the woman in the elevator who shows Craig how the 7 1/2 floor works.
Throw It In: The scene where Malkovich is hit in the head by a drive-by beer canning wasn't in the script, and was the result of a drunk extra having some fun while the camera happened to be rolling. Director Spike Jonze found it hilarious and left it in the finished product, earning the extra a SAG card in the process.
What Could Have Been: If the movie itself made you raise an eyebrow or three, check out the original version of the script. Your head could conceivably explode trying to fathom it ever working on screen. And the movie had Malkovichworld, so that's saying something. Additionally, had Malkovich turned down the role, the film would most likely have been titled Being Steve Buscemi.
Specifically, the original script involves Craig cross-dressing in various female puppeteering jobs, the Great Mantini challenging Malkovich to a dramatic acting duel, Lester leading a Satanic cult bent on using Malkovich to take over the world (and succeeding), and Lotte marrying the chimp.
Charlie Kaufman said that at least one studio offered to fund the movie had he changed it to Being Tom Cruise. Might cross over into Hilarious in Hindsight, if one considers Cruise's behavior from around 2005-2008.