> I have a shot of an actor running at the the camera. I have a
> key explosion and I want to put it behind him.
First of all, rotoscoping (drawing a mask path around the actor on each frame) is going to be tremendously tedious. Is it possible to reshoot with a green screen (or blue screen)? The time to reshoot can easily be less than the time to rotoscope a shot.
But, if you do need to rotoscope, you just need to animate the mask's path from one frame to the next. You animate mask paths in much the same way that you animate other properties: set keyframes for the Mask Path property, set paths at each keyframe, and After Effects will interpolate between these specified values.
Actually, it's usually easiest to have a separate mask for each major part of the subject that you're trying to isolate. Scott Squires has some great videos showing how to do this. They're linked to from the "Editing and animating shape paths and masks" section of After Effects Help.
Just fyi, but you can also go to lynda.com and get tutorials on this subject. :) if you do net searches (provided adobe doesn't already have this in it's base, or even it's video tutorials which are very good) you will find many things on this subject. If Adobe is exhausted as a resource, then you can use http://youtube.com and look up after effects and masking techniques as well-an excellent resource at times.
Hopefully this will help:
after effects videos
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Roto-ing live footage in AE is a pain. If the shot isn't that long you may be better off loading a copy of the footage in photoshop and make a version using ps tools where everything except the actor is erased, then drop that on top of the explosion and your plate.