Dawson Casting: Averted; all of the teenagers are played by actors between the ages of 13 and 15; Casper's voice actor is 14 with the actor portraying his briefly restored body just about pushing it at 17.
Deleted Scene: In the mirror morph scene, Dr. Harvey was originally also going to get changed into Steven Spielberg. The cameo was actually filmed, but cut from the film for pacing reasons. Spielberg was actually very happy about this, feeling that he's not much of an actor and was quite nervous in front of the camera.
There's also a musical number by the Trio called Lucky Enough to Be a Ghost, which was cut because the special effects needed to create it would've blown the movie's budget right out of the water. It can still be found on the DVD in two versions; the first which includes plastic models of the Trio that would've been used as a guide for the animators, and then a second version of Dr. Harvey on his own which would've been used in the film had they not cut it.