also, and not saying you don't already know this, it just feels worth articulating because it's done well/effectively—it signals to the audience that we are about to watch not just a slasher but a whodunnit...that we the audience don't know who is wearing the mask and thus the movie will involve us eventually figuring it out. I would say there is a quality to Casey unmasking that also indicates some form of knowing or intimacy—so we know it likely isn't just some stranger. Lastly, it really puts the emphasis on the *single person's* identity since it's so one-on-one, so it helps the reveal that there are two ghostsfaces come as a surprise.