You know those films that star the same person as two characters. Ever wondered how to film those scenes where the two characters meet? If not, you're probably reading the wrong page. Assuming you can't cheat by finding identical twins to act in your film, you are left with a few options, some less practical than others. Firstly, you could just make you never need both characters in shot at once, but that's boring. You could use chromakeying, but it's hard to make that look good, so don't unless you really have to. | 
