Hey, nice one alexgc. You could also do basically the same with some added random movement with a turbulence field. That's what I like to use. Here's an example which I did in the early phases of my...

Hmm... I'm having some trouble understanding what exactly are you trying to achieve... Will there be fog already in the room or is the actor "spawning" the fog?

Anyway, keying matte for the actor...