Danny DeVito quite literally saved It's Always Sunny In Philadelphia by joining during the second season. It's Always Sunny In Philadelphia season 1 debuted on FX back in 2005 and focused on the four friends who run Paddy's Pub; Dennis, his twin sister Dee, Charlie and Mac. The show was intensively low-budget and reveled in the darkness of its "heroes," though most of the humor came from how their own prejudices or stupidity often comes back to bite them.
It's Always Sunny In Philadelphia was an antidote to the frothy, harmless sitcoms of the era, and its popularity slowed grew over time. The series has managed the impressive feat of becoming one of the longest-running live-action sitcoms of all time, beating the likes of Cheers. Season 15 was confirmed in May 2020, and it's likely the show will continue so long as the cast and crew have fun making it.
It's Always Sunny in Philadelphia has been running so long that it's almost easy to forget Danny DeVito's Frank Reynolds didn't appear until the show's second season. Frank's awfulness almost eclipses that of the rest of the Paddy's Pub gang, and the actor has shown himself quite willing to go through endless embarrassment playing the role, such as crawling out of a leather sofa completely nude. Word of mouth surrounding It's Always Sunny in Philadelphia only grew following Danny DeVito's debut, but it turns out he helped save it in more ways than one.
The first season of It's Always Sunny in Philadelphia was not a hit for FX, which wasn't helped by its timeslot or the lack of any real marketing. The show's creator Rob McElhenney - who also plays Mac - explained during an interview for The Late Show with Stephen Colbert that when network president John Landgraf suggested adding Danny DeVito to season 2 for a little star power, he rejected it. McElhenney and the rest of the cast felt the strength of It's Always Sunny in Philadelphia was that the cast was made up of nobodies and including a star would throw that balance off.
He quickly changed his mind when Landgraf stated the series would be cancelled otherwise, so Danny DeVito joined It's Always Sunny in Philadelphia in season 2 as Frank and has been a key ingredient ever since. In hindsight, it's hard to believe the creative team flat out rejected the idea of DeVito joining, and it was only with the threat of it being canned that they changed their minds; it's fair to say it worked out for everyone.