The 'Piece of Me' singer appeared as Sara's camp BFF Abby on Thursday night's show.
The 'Watch What Happens Live' host has a good sense of humor about the situation.
The 'Heartbreaker' songstress shared a TBT of herself with Spears.
ET's taking a look at a few of the big name celebs who have been picked from the crowd to participate in Britney's now-infamous 'Freakshow' number from her 'Piece of Me' show.
The Bravo host attended Spears’ NYC show on her ‘Piece of Me' tour.
The 'Piece of Me' singer performed at Radio City Music Hall on Monday.
The pop princess did a three-night stint in Atlantic City, New Jersey.
Britney Spears' boyfriend of two years, Sam Asghari, didn't always have a picture-perfect physique.
Sam Asghari opens up about his relationship with the pop princess.
The pop star sits down with exclusively with ET to talk about her successful career and new projects.
Spears kicked off her first full-fledged tour in seven years and sits down with ET exclusively for a revealing chat.
The pop star is bringing her two sons on the road as her "Piece of Me" tour gets underway.
The superstar dishes on her latest beauty launch and what inspired the bold scent.
