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.
It might be worth it to consider what tone you, yourself, are placing on the text you’re reading. Words being spoken in anger or not will look exactly the same when they’re written down.