When Frank (Danny DeVito's character) is having dinner with his old sweetheart at the upscale restaurant, he makes a pass at the waitress. The waitress, Jenny, was played by Danny Devito's real-life daughter Lucy DeVito.
The vocals for the club music that Frank dances to were actually done by Rob McElhenney and Glenn Howerton. The song itself was influenced by an ad-lib Danny DeVito did when he started dancing and singing "go for it."
The classical piece played in the background in the scene when Dennis enters the bar after being phoned by Charlie is "Dance of the Sugar Plum Fairy" from Pyotr Ilyich Tchaikovsky's The Nutcracker ballet.