In a piece I'm working on, I'm working on a scene where the foreground would be frozen (as if in time) while a figure in the background is active. How is that accomplished? The opposite is fairly straight forward, but...

Many thanks from this naive...

You would shoot your foreground scene, cut out the background, and lay it over the top of your new background. Since you don't mention what programs you have, you might need to do this differently-

Take a single frame, and export it into photoshop. You will then paint over everything that is supposed to move. Use a single color that you can use for a chroma key. Then, lay it over top of the background, and key out the colored in portion.

You will need to extend the length of that single frame to match the length of the background video.

Doh!
Of course. Thanks a million Keith.

Take a single frame, and export it into photoshop. You will then paint over everything that is supposed to move. Use a single color that you can use for a chroma key. Then, lay it over top of the background, and key out the colored in portion.

Instead, while in Photoshop, erase the areas you don't want and save in photoshop format (or anything else that can keep an alpha channel). Just import that file in your NLE. There is no need to do a color key, as the areas are already transparent.

Also, instead of starting with a video frame, you might be better off starting with a digital still photo, which would give you better resolution, options for reframing, and no interlacing artifacts.

If you are going to use a matte, like J-F describes, then be sure to add the grain back in otherwise it will look suspect.

Thanks all. I think for my purposes Keith's approach will be the best because it will be easy enough to film the area twice as long as I block it correctly.