 the other half, where Bilbo is  seated, is further away from the camera, and stays put. It can be  seen that this gap between the two table "halves" is concealed  behind cleverly placed, bottles, plates, etc. Then, when Bilbo is  about to pour hot water in the teapot, Gandalf lifts the teapot lid  for him. The lid is not on the teapot, but placed on a spike nearer  to the camera to create the forced perspective. It is also clearly  visible when Gandalf puts the lid back.