One interesting variant of disguising an actress is the mask trick. Done well it can be very effective and captures the essence of the art of illusion and the necessary suspension of disbelief that lies at the heart of movie-making.
The mask trick comes in various forms but typically it is truly a trick: where one character impersonates another the actress may never be disguised at all, but the apparent putting on or removal of a mask hides a cut between the two actresses (the usual Mission Impossible trick). In recent movies, eg Charlie’s Angels, the trickery may be assisted by CGI effects.
Alternatively, the actress may be disguised with prosthetic makeup which is then apparently pulled off like a mask as in The Associate and the pilot for F/X: The Series (even though it’s been shown being applied piece by piece!) – or we may simply be shown a discarded mask as in Penelope.