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 would be the first thing to do so the fog can exist and go around him/her. Then I would propably track the features on the actor which will affect the fog. From that point on it's just a matter of trial & error or research & development as some are calling it . You just have to think though or try every method that could work. Eventually one method will survive. It's like cooking you know . You never know for sure what the end result will be

I believe both particles and fluids could work. You just have to try yourself. I'm not so familiar with fluids but with particles you could use a turbulence field to get the general whispy look. Then you could use air fields (based on the wake preset) attached to the actor to stir the particles more...