Well, you have the advantage of surprise
The scene doesn't know your shooting for visual effects, so that works to your advantage with camera placement. Be sure and try some extreme and odd angles.
Be sure and visualize the entire sequence, not just the shot. For example, if you shot only a few shots with the background in view, and in those shots there's little movement, then the visual effect becomes easier.
Say for example your scene calls for two guys walking and talking as they approach the bombed out city..
The wide shot, your BIG money shot, has the camera directly behind them, they are walking over the crest of the hill, the desolate city rises before them.. ..we just see the silhouettes against the skyline, feet going up and down.. (not too hard to use a portable green screen or rotoscope..)
Now you cut to some classic over the shoulder dialog.. as this is close up and shallow DOF, the background is all blurry anyway, and isn't showing the city in the background, maybe just a hint of it..
Now the next shot the camera faces the actors directly (the city is NOW BEHIND the camera) and we can see the actors reaction to the city.. not the city it self..
then the camera switches to show what the actors were looking at.. from the POV of the actors we see the whole wasted city in a panoramic slow pan.
In these shots, only the wide shot has the actors and the city in the frame at the same time..
as an alternative I wanted to suggest something different for the big money shot above.
Say we have our actors walking UP the hill, but they are not over the top yet.. say just 3/4 the way up the hill.. we KNOW they are going to go over the hill soon.. but we don't really need to show that.. this makes the masking and special effect SUPPER easy (relatively speaking), just replace whats "behind" the hill with your great set graphics...
If you did this, then you NEVER have the actors and the effect in the same part of the screen a the same time.. which makes this much more practical.
In another shot we have the actors walking down the center of the deserted street...
Find a high spot, like an overpass, put the camera there. Picture this shot like an isometric video game shot.... The actors stay in the center of the street, plenty of payment between them and the side walk, set up a wide frame so we see lots of the real city... (we replace the real later...)
Now in post we can just MASK a largish area around the actors walking on the REAL street and replace the rest with our graphics... again no green screen required, just good subject placement in the real world. The key is to try and NOT have your actors and the special effect in the same part of the frame at the same time..
(See the videocopiolt.net tutorial on set extension... )
Oh, and the moving green screen should work to!. You just need some frames and some bodies to haul it around.
