I didn't realize he/she was in that

Tom Selleck, Dabney Coleman and Erik Estrada all in bit parts in 1976's "Midway." Selleck was an aide on screen for a blink, while Estrada was a pilot, call sign "Taco."

EDiT: Sorry, it was "Chili Bean." Knew it was something not-very PC.

 

Laura Linney and Ving Rhames: "Dave."

 

Giancarlo Esposito (Gus Fring in "Breaking Bad") was the LAPD Narcotics cop (the one who first picks up on the Keyzer Soze angle) in "The Usual Suspects."

 

Ted Danson played an attorney who liked to dance in "Body Heat."

 

The guy was also a regular on Spike Lee's movies. And played the black cadet in Taps.

 

NBC all-stars:
Scott Adsit in The Italian Job and Accepted. Shirley from Community in Tropic Thunder. Mark Christopher Lawrence in T2 and The Island.

 

Mad Men has the girl from Community and Flo from Geico in its credits.

 

He played Buggin Out in Do The Right Thing.

 

Flo was also in the Ben Stiller version of "The Heartbreak Kid" as one of his love interest's family members.

 

There's the four musicians in Bill & Ted's Excellent Adventure:
-- Clarence Clemons, Martha Davis (of the Motels) and Fee Waybill (of the Tubes) as the Three Most Important People in the World and Jane Wiedlin (of the Go-Gos) as Joan of Arc.

Also don't forget that Huey Lewis (of Huey Lewis & the News) was the faculty member who told Marty McFly his band was "too loud" when they did an instrumental "Power of Love"

 

Thomas Lennon (Lt. Dangle from Reno 911) was the doctor in "Memento", the one giving Sammy Jankis his memory tests.

 

Laura Linney was in Searching for Bobby Fischer, too.