Hi Joe
There are a few options in AE for this.
You will need to Key (if shot with green screen) or roto the footage to define the actor. Then I would animate a number of null objects (the more the better) to follow points of interest on the actor which will be areas of fire emission.
You can then use CC particle world or better: Trapcode particular (if you have it) to generate a flame who’s emitter follows a null (repeat for each null)
If using CC particle world you will probably find best results with the bubble particle shape in combination with a CC vector blur and a little extra vertical blur however I find this result only marginally acceptable. You can augment this with an Alpha track matted precomp (with particle effect above) containing a few multiplied layers of animated fractal noise. Add a bit of CC toner and you could end up with something half decent with some tweaking.
Trapcode particular will be capable of a much more convincing fire effect with added ability to have the flames avoid a user defined spherical field which you might be able to use to make it look like the particles are rolling around arms/legs etc…
FumeFX or any 3D package based particle/fluid software will do a much better job however you will need to create some sort of 3D character for the particles to react to which also has to be animated to copy the movement of the actor.
Personally I would try a quick Trapcode particular test to see the quality of the result and if not satisfactory I would move over to Particleflow and Afterburn in 3D studio Max or use the new PhoenixFD simulator.
Graham Macfarlane
3D animator and VFX specialist
London UK