Sometimes the plot of a film requires a character to change their appearance or disguise themselves as somebody else. Often for this to work, it requires a voluntary suspension of disbelieve on the part of the audience: either the change is so subtle that the character ought to be recognised or the makeup is so heavy that it looks false – not to mention all those times someone has run their hands through her hair without noticing that it was a wig!. But hey it’s fun.
One variant of disguising an actress is the mask trick in its various forms. Done well it can be very effective: the actress is never disguised at all but the putting on or taking off of a mask hides a cut between actresses (the usual Mission Impossible trick