I had never really noticed before but when the camera is filming Melvin's close-up Larry is still in the scene and acting right along with the child actor. As Melvin tells about the lady who came out of the bottle, Larry actually looks in the bottle. You can see him move by the shadow on the wall.

https://i.postimg.cc/X7ByYXKn/kid.jpg

https://youtu.be/ziUOqhXRy3Q?t=142