Search found 1 match: Mel Brooks

First one I thought of is Hail Caesar, but that's cheating a little as they are filming a musical but other than that it fits perfectly.

That made me think of The French Mistake scene in Blazing Saddles, but no main cast members are in that, but it reminded me of the I'm Tired cabaret scene earlier in the film so this probably counts. Then I realised that Mel Brooks does this all the time. Men in Tights was mentioned, but there's also History of the World Pt 1 (Spanish Inquisition) and Young Frankenstein (Putting on the Ritz) and of course Springtime for Hitler in The Producers

Outside of Mel Brooks, Justin Timberlake and dancing girls lip sync to the Killers in Southland Tales, and I think I recall a big cast dance number in Clerks 2, but I suspect there wasn't any cast singing so both films probably don't.

Hocus Pocus? (I put a spell on you)

Knights of the Round Table in Holy Grail?
Always look on the bright side of life in Life of Brian? (Heads bobbing counts as dancing right?)

Gremlins 2? (New York New York)

Hudson Hawk ?(Swing on a Star)