The most common way (only way i know, lol) is Using a blue/green scene (chroma).

They are filmed seperately then keyed in later

http://filmind.meetup.com/296/?gj=sj3

I've done this a fair bit in films where we're limited with the numbers available.
For example if you want 2 people leaning against a wall, facing the camera, with the two different people played by the same actor. The easiest method is to lock down the camera on a tripod. Then shoot one of the people leaning, then shoot the other (with the actor changing clothes etc).

In post you just crop each shot and layer them on top of each other so it looks as if there are two different people in the shot.

Very simple to do, just make sure you don't move the camera or change the lighting. Also be aware of the actor's shadow.

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There's daggers in men's smiles

Thanks for the tips guys.
am working on a very tight budget($0). The shot am trying to take involves the characters overlapping each other at one point. Is it possible?. Also is there anyway I could do the act with the camera moving?

Your gonna have to use greenscreen if you want the characters to over lap. And I don't know any free chromo key programs but, theres lots that cost something. And unless you can do the perfectly same movement with the camera 3 times in a row you can't do it with a moving camera.

H.A.

The only way to do it with a moving shot is to have an automated machine making the move that can replicate it perfectly. And, if you download the right plugins, you could greenscreen with anything, including Windows Movie Maker.

"We all have the potential to be great. It is our inability to do so that makes us miserable." C.S.Lewis

thanks man.

What if someone or something which is considered as a part of the background moves. what would happen then?

Well if the background moves then it moves, I dont get your question.

H.A.

It doesn't matter if the background moves as long as nothing crosses the matte line between the duals. it could be possible to carefully matte around one of the characters and have a fully functioning/moving background behind the other but this can be really tricky. The easiest way is to divide the screen and make sure nothing crosses from one division to the other. This can be done easily if the framing is exactly the same (using a tripod as mentioned earlier)

RJSchwarz
San Diego, CA

Thanks rj

If you're on a very tight budget and really need to make it work, you could use After Effects to mask out everything except the character then overlay that over top of the original shot. But I hope you have ten thousand hours to spare