What particle system are you using? If your particles are fading out then make the lifetime longer.
It sounds like you're on the right track. Adjusting the physics of the particle system is key to making this work. The second think you have to do is match the color grading. The third is layer it with the head, the explosion particles, and the background. I'm guessing that you'll need 6 to 10 layers to pull it off with your particles somewhere in the middle.
I can't give you more guidance until I know what particle system you're using.
Thanks for your reply. When I first tried it, I used CC Particle World. I wanted approximately 30-40 balls to come shooting out of an exploding head. I set up a layer for the background, a layer for the head (well, the person with the head) that I color-keyed, a layer for the explosion (stock footage that has a black background that I blended using Lighten). and a layer for the balls. As soon as the explosion started (actually, in the next frame) i wanted the balls to appear to burst out and fly in all directions and out of the frame, some getting larger as they fly toward the camera. The problem is that some moved slower than others, and some didn't ever leave the frame, they simply disappeared after the three seconds (the length of time I wanted them on the screen because it's slow-motion).
After I couldn't get it to work, I tried CC Ball Action, but I can't get them to do what I want them to do, i.e. get larger as they fly toward the camera then fly out of the frame in the alloted time.
Another problem is that I really need the balls to look like Trix (the cereal) and not 3D-shaded smooth balls. Once I figure out how to get them to react the way I want them to, is there a way to use, say, jpg files of the Trix instead of the pre-packaged balls?
I know there's probably a way to do this with some expensive plug-ins, but I don't want to spend the money because this is just a short film. Any suggestions?
Thanks again for your reply.