You are umpiring at the striker's end (square-leg). As the bowler runs in to bowl, you see the wicket-keeper is running up from where he was standing to take up a new position right behind the stumps. What do you do?

When the wicket-keeper moves up significantly behind the stumps while the bowler is running in, this is an unfair practice designed to deceive the batsman (Law 20.4.2, Law 41.7.1). The umpire must call and signal Dead ball immediately and warn the fielding captain. The movement happens as part of the bowler's approach, making it unfair.