Nothing screams classic spy flick than a good old fashioned mask rip.
You know the ones — some nameless henchman walks about a secret bunker only to turn and decimate a bunch of armed soldiers who can't shoot, then pulls off a mask to reveal that he's in fact the protagonist in disguise. Mission Impossible had a whole mess of them.
There are quite a few tutorials online that walk you through how to pull off this effect, but RocketJump Film School's Freddie Wong shows you how to do it with only the built-in tools in After Effects, no plug-ins or additional software required. Check out the tutorial below:
Even though Wong does a great job walking you through the process step by step, this tutorial is meant for more advanced users who know their way around the AE platform, especially when it comes to the tracking, rotoscoping, and puppet tools.
One thing to keep in mind, whether you're a seasoned pro or a total noob, is that lighting plays a huge role in getting this effect to look right. Make sure the lighting in all of your shots are the same, because if it isn't, it won't matter how great the effect is executed — the two shots will look completely different and will ruin the shot altogether.