It would be easier to do it in After Effects, for sure. But, if you really wanted to do it that way, you could just go into the Transparancy option, make sure the matte fill-in color is black, then move the sampling area in the upper-right hand of the window to just show that one person. If you want it to move (travel), then you'll have to make it keyframable and animate the box movement that way. Hope that helps you out a bit.