well-executed, convincing greenscreen composites require A LOT of skilled work... and if the bulk of the work doesn't occur at shooting/lighting then it will have to occur in post, usually in twice the quantity.
i have heard from a compositor that worked on sin city that it was a nightmare job. from what i've heard, there was an absurd amount of rotoscoping needed.
also keep in mind that doing greenscreen composites for a final b/w image is a lot easier than finishing to a color image.
also, the idea of shooting greenscreen actors to match with existing locations is going to be very hard to do because of the difficulty in matching up lighting and the subtle organic color reflectance/bounce generated by the location (the achilles heel of grscr compositing). when the locations are being created via CGI, they can adjust the lighting to match the footage of the actors. also, camera moves and even panning/tilting will add great complications (motion tracking & photogrammetry will be necessary).
but if you do go ahead with it, two things that will save you loads of trouble (and despair) will be to avoid showing the actors' feet touching the floor and to shave their heads or have them wear hats.
not to sound like a downer, but it's almost certainly not worth it unless you yourself are an ace compositor and know how to plan/shoot around the limitations.
Edited by Jaan Shenberger, 13 September 2006 - 07:27 PM.