Having one character move through the frame while all others are imobile is a way of drawing attention towards that particular character....in much the same way that you'll notice main characters walking down a crowded street will often be wearing brighter clothes than any of the extras who are there simply as "filler". If he had wanted to add a prequel alien to the thing to make a link between PT and OT then he should have had the dug relaxing in a corner like everyone else. A battle droid head in a pile of scrap is a good example of this.....its doing what the rest of the junk is doing....nothing. And your attention is still on the subject of that particular frame....the red droid that later blows his motivator