I've been wondering about this for a while but never really bothered looking into it but I was watching this video today and I thought it was time to figure it out!
Go here and skip to 1:50
As you can see the trololokl guy is comletely gone from the frame even though the space taken up by the people in the shot is smaller than the original guy. which means that simply keying him out and putting something else in isn't all you have to do! I've seen this several times in fake trailers and such and I'm at a loss to how you do this.
the trololokl guy