Setting someone on fire via after effects
 

Is there any plug-ins or pre composited flames that I can use to show someone on fire??

Look into ParticleIllusion (www.wondertouch.com) or trapcode Particular (www.trapcode.com)

I got particle illusion I'll give that a shot, I'm not that familiar with it but hopefully It's not too hard.

What exactly are you trying to do? Will flames be coming off of them while they are moving and swinging around? Are you trying to sell it as a real effect?

Definitely, we'll be trying to sell as a real effect, what I've got now with particle illusion it won't look good if the guys waving his arms around so he might have to be on the ground, do you know of a way he could be moving his arms around and have realistic looking fire?

You will have to do a track.

1st tip: make the clip short.
2nd: take a look at these Cow tutorials by Jeff Bellune: Igniting a Soccer Ball, part one & gniting a Soccer Ball, part two. The first will show how to correct for camera movement, the second one shows how to track motion. Third one is expected in a week on how to composite things in AE.

George/

The soccerball is one thing but a human with a dead on real looking fire coming off him is another. You would be better suited to matching movement of the character in a 3D program. Here's how I would do it:

1. If camera is locked down, handheld is fine too as long as person was standing in one place and not moving around, track in 2D program to stabilize the shot. Export an image sequence out of this - know your camera setup from the shot.

2. Bring image seq into 3D program and map out a rough character model in motion to match your characters movements.

3. Place a dynamic sim overtop of the areas of that person that need to be on fire. If there are clothes flapping around and whatnot, this can be that much harder.

What you'll get out of that though is true movement to the flames ina 3D space. If he spins, the flames will react correctly and also as his arms move independent of himself it will look correct too. Render out in a few passes to get smoke and flame and composite those in. Good example is in Kingdom of Heaven, most of the characters you saw on fire in the last battle seq were done this way, closer the camera. Farther back and they were completely CGI.