Usually top lighting in front of the scrim will achieve this effect.
Front lighting will sometimes shine right through it, so thats something to try and avoid. Usually, the steeper the lighting position, the more opaque the scrim is.
thats is true. if you want the audience to see through the scrim, light it such a way so that the light is all behind (stage side) of the scrim and no light hits the scrim. to make the scrim opaque a high top light is good on the audience side of the scrim, or with front light it usually makes the scrim opaque (though if there is something white behind it, that will sometimes show through).
if you want the audience to see through it and have spooky sort of shadows, you can stick a couple of par cans or pc's on H stands behind it on the floor a fair distance away and behind the action. this makes the shadows go bigger!
As someone mentioned before, be careful if you're using a white cyc behind the scrim, because it is hard (not impossible) to hide no matter how you light it. If you are front lighting the scrim, they will bounce off the cyc and defeat the purpose. Also, the more still it is behind the scrim (the fewer people moving around for scene changes) the more dramatic the reveal will be.
Using a lighted cyc behind a scrim does make a cool effect though...
At my high school we are doing My Fair Lady. The director is well experienced, and has done many plays, including Music Man at the Playhouse. But she has very little technical knowledge. There is a wall bolted to the concrete floor that starts extreme US. Three feet behind that is a track for sliding in and out canvas flats, since our celing is like 15 feet high and we can't fly. Right behind that she wants a blue scrim, which is apparently going to be the only thing between the audience and the entire backstage. That being the case, seeing through is not exactly what I had in mind. I am going to try to convince her to put something up behind the scrim, now that I know that any extra light will make it somewhat transparent. This show is is running real tight, with only 2 dimmers left for specials, and "expensive," at $300.
I gather you already have the scrim and that is why you are using it?
Otherwise you don't need a scrim if you are never going to see behind it. So if you have options, just use a blue drop or light a cyc blue or whatever colors you want. If you have strip lights, this will allow you to change the color, look, and mood for each scene.
Good luck. I did My Fair lady a few years ago at a theatre without fly space and we used turntables to facilitate fast changes between Higgin's study and the various exteriors.
The flattrack has 2 slots that will hold a total of 6 huge flats at a time. The flats will be painted on both sides, and when put in the right order will make one huge backdrop. For certain scenes, all of the flats will be removed revealing the scrim, which we already have. All I have to light this would be 3 8' boreders, if I can get them. Otherwise, I'm screwed. :? If anyone is interested, I made a Microlux Light light plot that has the locations of where everything is.