Here's a screenshot from the 2014 Marvel movie Guardians of the Galaxy. Star-Lord and his buddies enter prison and are immediately greeted by a huge blue alien that wants to get frisky with Chris Pratt's character. The scene lasts only a few seconds--not long enough for fans to realize that the alien is portrayed by Nathan Fillion, the actor who played Mal Reynolds on Firefly.
This 1 of 10 movie cameos that you've probably missed. Watch the rest in this video by Screenrant, including unexpected roles in Spaceballs and Hook.
-via Geek Tyrant