The Breaking In actress revealed that a script for the film is currently being written and opened up about a potential storyline for her character.
"I will be doing the Party Down movie," Mullally said at the TCA press tour. "John [Enbom] is writing it right now."
She continued: "I think we're going to see Lydia's ex-husband - Ed I think is his name - who was really racist and a misogynist."
The 53-year-old added that the entire cast of the TV series is expected to reunite for the film, with the possible exception of Jane Lynch.
Adam Scott previously insisted that a film based on the short-lived Starz series was nearly complete, claiming that shooting could begin in summer 2012.
Party Down was cancelled in July 2010 after two seasons.