Shoot the background plate as you want it to appear in the final shot. That would be the bed with the “moonlight” falling onto it. Then shoot the actress on blue or green screen as you would normally do. Make sure that the actress is away from the green/blue backing so you can light her independently from the backing. It will be a bit tricky since she cannot lie right on the backing. You won’t be able to light her correctly if she is in contact with it. You will need to get some distance away and let the light mostly fall off of her while the light on the backing remains strong and even. Attempt to light her in the same manner that you lit the background plate she will be composited over. Light her from the same direction and use the same effects or gels that you used on the background. It will be a challenge to position her so she looks like she’s floating AND also keep a full green screen in the background. Don’t worry about shooting off of the backing. As long as she has some of the backing around her on the screen, you can garbage-matte out everything else.
It might be possible to build a supported platform just wide enough for her to lay in with pipes or something strong and thin to support this platform. At any rate, if lit correctly, the green/blue backing will drop out and you should see youactress over the background plate. Keyframe the actress and she will appear to float over the bed. You could add a collor correction filter after the chroma key filter and tweek the color to blend into the background realistically. Definitely going to be a challenge to do.
If she doesn’t need to move around at all (sleeping soundly?) simply shoot the frame with her in it and with the desired lighting and then use a graphics program such as Photoshop to “cut” her out of the background and use the resulting still as a graphic elememt with an alpha channel and composite her in over the background shot. Easy!
tom