The 47-year-old star had agreed to film a cameo for Will's new movie with Amy Poehler but when she arrived on set "four hours late", she left cast and crew frustrated with her diva demands.
Speaking to Andy Cohen on 'Watch What Happens Live', Will said: "One script note was like, 'I don't want to do this scene,' even though it was totally approved ahead of time."
And Mariah was supposed to sing a song but, according to Will, "it never happened. It became quite a kerfuffle. A.k.a. a s**tstorm."
He went on to explain that Mariah decided she wanted to sing a different song but producers did not have the rights to it.