For thems wot don't know: The throne's been usurped, and Danny Kaye, an ex-theatre entertainer, is sent into the King's castle on behalf of the Black Fox (read "Robin Hood"). Kaye's taken the place of the Jester newly appointed to the castle, unaware that the Jester was supposed to be there as part of someone else's furtive plans... A witch casts spells, changing Kaye's personality occasionally, and Comedy Ensues.
(The Xena episode For Him The Bell Tolls paid homage to this movie, where Aphrodite casts a spell on Joxer so that the haplessly genuinely becomes Joxer The Mighty when he hears the sound of a bell. And changes back again on the same trigger.)
I'll have to watch it again - not least of which because of the songs. Oh yes: It's a musical. And writing as a filker, they're good - lyrically, certainly. Lots of clever wordplay in there.