Nicole Richie
is a big prankster ... but did one joke on her father go too far?

Richie, whose new show "Fashion Star" premieres tonight on NBC, was a special guest on "Watch What Happens Live" with Andy Cohen on Monday night.

During her visit, a couple of her good friends phoned in, asking what was the best prank she's ever pulled on someone.

She had an answer almost instantly -- and the joke involved her father, her stint in rehab and cocaine.

Watch the video above to find out what she did!

Hot Videos