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.
The normal method is to use a split screen, with one character in each half of the screen. There are a few things to watch out for when filming a scene for this. Obviously, the camera cannot move (unless you have one of those programmable camera cranes that can repeat movements exactly. And then the wheels of the crane can't move. The other important thing (and this is equally important if the effect is going to work well) Is that the lighting has to remain constant throughout the whole shoot. After all, no room in the world can be brightly lit in one half and dull in the other without some barrier in the way. A change in light levels will lead to the the dreaded line - the visible join that gives away how the effect was achieved. Of course the way round this is to have a feathered (gradual fade) wipe rather than a straight edge. The problem with that is that it makes the next point trickier. You have to make sure that the actor never strays into the other half of the picture (as bits of them will disappear).
To produce other camera angles to cut to, a lot of people like to film one character over the other character's shoulder. This is very easy to do (because you cheat). The character whose face you can see is played by the usual actor, while the shoulder is supplied by somebody of the right height, build, etc wearing the other character's clothes.
To make that scene where the two characters are wandering in and out of the same room but don't quite meet, the basic techniques are the same. You will probably want both characters to have full use of the screen space if you only see one at a time, but this is no problem (as long as there is no one else there - that complicates things a little). As before, you need to keep the camera and lighting the same for the whole shoot (you can actually get around this to some extent if you film everything in chronological order). Anyway, all you do once you have shot all the segments you need is to cut between them. As long as the camera and lights did not change the joins will be invisible. Again, as above, you can get around slight variations in light by fading between clips. To deal with movements in the camera you will have to cheat by using a cut away shot, which may spoil the over all effect, but you will probably want other camera shots for a long sequence anyway.
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