How to reveal one layer with another layer in After Effects
If you're trying to an effect like this: / bts0dbrdft3
Then here's what you need to know:
Top layer: someone shot on green screen, keyed, with posterize time.
Middle layer: foreground art, using top layer as an alpha matte.
Bottom layer: background art!
If you're trying to find an affordably priced spy comedy delivered right to your door, then all you need to know is / notart