Thanks for your answers, guys.
I want the reflection to just stay and watch for a couple of seconds, and then cut. Nothing fancy, just need to use some sort of masking.
I figured it would be better to do this in the laboratory than having to match "frame accurate" the actors' move up to the point in which he leaves or just watches.
My question was if you guys know whether this is possible to do in the lab or if I am stuck with the mask filter option. I would like to be able to shoot both versions full frame, and using only half of each in the final shot.
David, you were saying something about the actor leaning out of the reflection. That can be done, no problem there. The camera will be in lock-off position, so no motion tracking is involved. Also there is no superimposing involved between the character and his reflection. But can I link the two shots together in post production, without using film printing?
In this region the film printing option is... out of reach, at least for a student like myself.