If you're going to do the classic Mission Impossible gag where a character suddenly yanks off a clever disguise to reveal that they were actually another character all along, it is imperative that the disguised character and the revealed character be played by the same actor.

Otherwise, this effect is not cool, but extremely cheesy.

You can't have Jennifer Garner yank off her Sydney-mask and suddenly have Melissa George standing there. That's not Mission: Impossible, that's Scooby-Doo.