I am very new to after effects so I appologise if I am using the wrong expressions or you will have to explain things in a very dumbed down way but I am trying to re-create this rene magritte painting with video clips. Is there any way I could use the same footage to mask my actors back into the mirror itself?
Easily done, but you don't use any mirrors at all. If you watch making-ofs of films that use such tricks like certain horror movies, you will see that it's either done with stand-in actors wearing prosthetics, make-up and identical clothing or simply using green screen/ blue screen. All comes down to the specifics of a shot. And there you pretty much have your answer - shoot yourself in front of a screen, shoot the mirror frame with a screen fill and ekey out both, then insert your layers. Of course you can do it after the fact with traditional maksing just as well, but in your very dark shot you'll realyl end up having to trace everything by hand frame by frame as any form of semi-automated function, be that Rotobrush or the mask tracker will struggle to do anything. Anyway, I suggest you read the online help and research the web for a tutorial based on what I explained.