How does the phantom of the opera disappear in his chair?

Answer 1

The Phantom is a true magician, he cannot reveal his secrets, he made an oath.

If you mean by stage, a simple trapped door, and smoke, the actor that is playing the phantom simple opens the trap door and goes down the steps. It makes it seemed he had disappeared.

Trick Photography now you see him and now you don't. This is what was used in the movie. They film the Phantom sitting in his chair, next they film an empty chair, they splice them together.