Reese Witherspoon is one of Chelsea Handler's closest celebrity pals, but in a clip from Sunday's all new episode of After Lately, the 35-year-old Oscar winner becomes her biggest nightmare.

The problem: Witherspoon enlists the help of Handler's writing staff to prepare a speech, but her perky demeanor grates on Handler, 36. "She is driving me crazy!" Handler complains behind Witherspoon's back. "She is so awful."

The comedienne and bestselling author ignores her employees' polite suggestion to confront Witherspoon, explaining: "What am I supposed to do? I'm gonna go tell Reese Witherspoon that I think she sucks? She's a mess!"

Will Witherspoon catch Handler badmouthing her? Watch the clip (above) to find out and tune in for an all-new episode of After Lately Sunday at 11 p.m. EST on E!.