Putting an object behind a moving person?

I have this snippit here from a short film I'm working on. In this scene, there's a homeless killer in the restuarant and he's looking backwards, holding the gun over his head and shoots a pillar. There's a gun flare effect you see on the top left of this image. How do I put that behind the subject? (in the wig) I've seen so many videos of just how to put text behind the object but not an object behind another object. Only help me out here if you know how to do it. Thanks!