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I would just lock the camera down, have the actor stand still at first then do whatever action is required. Offset the clips in post and mask out the mirror.
How would you pull that off on a budget? Something where the actor outside the mirror does something but their reflection stays the same? Is this cheatable with two actors? Or am I going to be forced to green screen the mirror and add in the other image?
A lot depends on how you block your shot. If you're doing a profile shot, well, that may need to be post (but a simple splitscreen can sometimes work rather than greenscreen, but you might also try difference matting instead of GS). If you're doing an OTS (over the shoulder) PoV on the character and his/her "reflection" is staring back into/towards the camera, the classic cinema trick is to not have a mirror at all: you have a mirror frame that just is a hole that leads into another room, with a 2nd actor facing the first. You can see a first-person example of this at http://www.youtube.com/watch?v=iX8iVo5vc8o.
If you can pull this off, you only need to match the hair and clothing of the actor...you'll only see one back-to and one face-forward. The width of your shot also may not require hardly any set matching; only match the items in the shot (this does sometimes include the floor, though). The two actors do need to rehearse like mad to pantomime accurately, as stated previously
Just an idea and is 100% dependent on your framing. But you have to love visual effects through set dressing.
I was going to do an over the shoulder shot, so I was thinking of using two actors in the shot. This might work.