Billy Zane -- You'll Never Believe Who Else Is in 'Zoolander 2' (VIDEO)

Billy Zane is just one of the big names signed on to appear in "Zoolander 2," and he let us in on another NEW secret cast member NOBODY has mentioned yet -- or at least, his friend did.

Zane was out in Venice, CA and our photog was beyond excited about the news Zane would be in the highly-anticipated sequel.

Billy was walking with a woman who just couldn't resist getting in on the convo, and that's when she slipped -- seemingly dropping the name of another star in the film. This person hasn't been revealed, but if Zane's pal knows her stuff ... this should be awesome!