Sorry to say, but the easiest way would be to have the actor be motionless the whole time, or moving very slowly. Another way you could do it is to composite the person in, and actually, it wouldnt be to hard, because since he's not moving, the masking wouldnt be bad! you would just have to make sure that nobody else occupies his space, maybe put a planter or something where he's supposed to stand to keep people walking around that spot.