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')
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