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).
The excellent – and entertaining – tvtropes categorises the various types of disguise as: Latex Perfection with full-head masks or prosthetics; Wig Dress Accent which is not as impenetrable but is usually, at least intended to be, plausible; and Paper-Thin Disguise which ought to be transparent to the other characters.
That said categorising these disguises for this website sometimes gets a bit tricky (eg in the pilot of F/X: the Series: Carrie Anne Moss is disguised as an old man; the disguise uses prosthetic makeup; the reveal is a mask trick; oh yes, and she appears in numerous other disguises during the series. And how do you categorise Jenna Elfman disguised as Richard Dreyfuss pretending to be a tribal leader from New Guinea (excluding the words disaster, politically incorrect, mistake etc etc that is)?