It's Always Sunny in Philadelphia is well-known for its characters who have a complete lack of shame and the show's willingness to touch on any topic. It also has some popular and easy-to-spot celebrity guest stars, like Jason Sudeikis who gets drafted as the fifth member of the gang or P-Diddy who appears as a "doctor."
However the show probably has more celebrity guest stars than most people realize. Some of them have such small roles that people barely register their presence. Others just look so different from real life and act so crazy that you wouldn't be able to guess who it is!
10. Guillermo Del Toro
Guillermo del Toro appeared in two episodes of the show, though he was highly unrecognizable. Dressed as Pappy McPoyle, and older member of the McPoyle family, he has a huge beard and long hair, crazy eyes, and he's constantly yelling gibberish. He is first seen in Season 8, Episode 3, “The Maureen Ponderosa Wedding Massacre,” where he briefly rants at Mac, Dennis, and Charlie. Later he appears in Season 11, Episode 7, “McPoyle vs. Ponderosa: The Trial of the Century,” as a witness in a trial. Here he is hiding the key evidence, a bird, underneath his top hat.
Del Toro is a famous filmmaker who has made hits such as Pan's Labyrinth, The Shape of Water, and Hellboy II. He also did Pacific Rim, which Charlie Day (who plays Charlie Kelly) worked on.