The “official” story that Rob McElhenney and Glenn Howerton tell people about how Danny DeVito ended up on It’s Always Sunny in Philadelphia is that ratings during the first season were so low that FX President John Landgraf literally forced them to cast DeVito on their show to improve ratings. After the first season, the entire cast was happy with the way things were going and FX loved the show, but the network insisted that they add a big name to the cast so that people would actually watch the series.
The gamble worked, obviously. The show is now in its 11th season, and has already been renewed for a 12th on FXX.
Unofficially, however, the cast had to agree to a certain, uh, provision to get DeVito on the show. Six years ago, an Always Sunny camera crew captured Rob and Charlie Day going over DeVito’s contract. Day was ultimately forced to provide oral services to DeVito while his wife, Rhea Perlman, recorded the encounter and Fred Savage watched while eating pizza.