Removing background from an image based on a clean plate of that background

I want to know if there's an actual technique for what I'm describing in After Effects:

-I have a video of someone walking past a brick wall, and I want to key the wall out so I just have footage of a person walking in front of a transparent background.
-I also have a clean plate of the exact same wall, no person in front.

Based on the data from the clean plate, is there some way I can use that to isolate it out of the video? Is there a way to take the information from the clean plate and remove it from the video so that only the person remains?

Just looking for some way easier than rotobrushing. Thanks.

That's certainly worth giving a try, but don't get your hopes up too high. The "easy" and definitely the most assured for success is using Mocha Pro ( http://www.imagineersystems.com ) as it has the Remove module. Ah, that link now takes you to Boris FX as they're now the parent company. In particular:

http://borisfx.com/videos/basics-of-the-remove-module-with-mocha-pro-part-0...

That's quite a price tag of course. But, it's doing what native AE can't (or at least very easily) as it can compare & select pixels over a time span and doing sub pixel manipulation to make things seamless.

A couple months back I had a tricky remove shot & tried for several days to see if I could do it "by hand" with just AE and Photoshop, and failed miserably. I'd pretty much given up on it when later in the week, Mocha Pro went on sale. Upgrading was on the wish list anyway, but the timing still seemed suspicious.

Ah your right, in which case the OP could just use the built in Mocha AE to assist in getting a roto matte. Probably easier than the rotobrush I'd think.