Three of the main characters were the same actor, and yet there are shots with all of them in the same scene. When I saw this as a young adult I didn’t even notice that Mike Myers was playing three roles and was genuinely dumbfounded years later when I found out.
How did they do those scenes so seamlessly?
Digital magic.
My dad and uncles made home videos on a 8mm camera doing this trick when they were teenagers, so it must be pretty easy even without a computer.
One thing I know is if the background doesn’t change, you can basically film one character’s part in the scene, then re-expose the film to do the next character’s and so on.