Rotoscoping. Now, if the person is on a chromakey stage you can use color keying.
Take this advice from Dave LaRonde to heart:
Dave's Stock Answer #2:
When you're out on a shoot, and you say, "we'll fix this in post" without knowing PRECISELY HOW you're going to fix it in post, don't shoot it! You'll only end up shooting it over again.
Since post typically costs three times the cost of production, fixing something in post is not a way to save money, but rather a way to spend more of it.
And, before you say "well fix it in post," always consider who's doing the work, especially if you're the one doing the editing.
Special effects like what you're asking about require planning in advance to make them look good. Lighting, color, angle (and more) must match between the two shots for the end product to look realistic.
- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')
No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.
Thanks Michael! Yes I have shot the person in front of a green screen, and have removed this persons background through chromakey. But now I want make this person partly or fulle empty and fill it with a movning picture. So I need to get the outline of the person. In Photoshop I would hit the magic wand tool to remove the inside of the person (that is totally black) and the background image would come through, but I need to know what the equivalent is in AE. The outline ofcause needs to follow the person when it's moving, so masking it by hand is not a very good alternative, but I guess i need some kind of mask... :)
You're halfway there if you've already keyed out the actor. Now all you need to do is put that layer on top of the video you want to show through the body. On the video layer, set it's TrkMat (track matte) to Alpha Matte. You should see this option to the right of the layer name. If you don't see it, you might need to click the Toggles Switches/Mode button at the bottom of the timeline, or simply press F4 on the keyboard.