"[It's] so exciting to be joining Fashion Police," she tells PEOPLE at KIIS FM's Jingle Ball in Los Angeles. "Of course, it's in the honor of my dear, dear friend Joan Rivers, and to take that chair means so much. I think I can hear her from heaven saying 'Don't hold back!' "
The decision to join the hilariously critical panel didn't come easy: Griffin, 54, reveals she needed the approval of Joan's daughter, Melissa, before getting on board, even despite her admission to PEOPLE in November that she and Rivers had actually discussed a possible part for her on the show.
"Melissa gave me the blessing, which was a deal-breaker for me – or the deal-maker," she says. "That was what made me go, 'If Melissa says it's okay, then it's okay.' "
But of course, Griffin's adoration and respect for Rivers's legacy doesn't mean she's going soft on the sass: "I'm saying all these lovely things now, but believe me, I'm taking everybody out!"
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