10. Matt Damon - Interstellar
Kicking off with the most recent surprise actor appearances, one so devious it stands next to saving humanity with love in terms of post-movie discussion points.
Dr. Mann, the head of the original expedition through the wormhole, is mentioned a lot through the first half of Interstellar, described as the "best of us", but remains conspicuously unseen until the Endurance crew land on his planet. It's only then audiences finally see that he looks like... Matt Damon?!
Damon was reported to be part of the film during the film's production, but in the build-up to release this was mostly forgotten, with the actor's name absent from all press releases, making his appearance a surprise for everyone on opening day. It has a strange impact, working more on a contextual level than really adding much to the film as whole; it's essentially Christopher Nolan proving that he can keep a massive secret from the masses in the internet age (as well as allowing for a shocking death scene).
Quite what the impact of the Mann reveal will be in the future remains to be seen; will Damon's role remain somewhat down-played, or will Interstellar become known as another movie with Matt Damon? Has to be the latter, surely.