This job will be easier if your camera was locked out and he was accurately in position. You need layers. You need a clean plate. Do you know what that is? A clean plate is a shot without your actor in the scene. I hope you have one of those, it will make life a lot easier. If you don't you can still pull this off.
Without a clean plate here's what you do:
- import your two shots into the project panel of After Effects.
- Double-click the first shot to open it in the footage panel
- Find a suitable in point and set it by using the keyboard shortcut Alt/Option +[
- Find a suitable outpoint and set it using the keyboard shortcut Alt/Option +]
- Select both clips in the project panel and right click or use File>New comp from selection from the menu and create a single comp from both clips
- Open the composition settings by using the keyboard shortcut Ctrl/Cmnd + k and extend the comp time to make room for the entire affect
- Put the first shot on the bottom layer and the second shot on the top layer overlapping them by a few frames
- It's time to check alignment of the two shots by using the blend mode "Difference" or "multiply" or "screen" to make sure both shots are perfectly aligned (if you do not see the modes panel in the timeline press F4 or click on the second icon at the bottom left corner of the timeline to reveal the molds/switches column
- If necessary adjust the position scale and rotation of one of the layers until things line up
- Return the blend mode to normal
- Go to the end of the overlap then dad five or 10 frames and split the top layer, the one with your actor dressed into two pieces by using the keyboard shortcut Shift + Ctrl/Cmnd + d
- You are now going to rotoscope or create a mask for the 10 to 20 frames where the actor overlaps himself
- The masking does not have to be absolutely accurate because you're going to be adding blower and glow to the edges
- When you have the actor separated from the background move this layer to the top in the timeline and extend the bottom copy to cover about half of the overlap
- Animate opacity, and blur, add glow, Try cc light burst and and maybe lens flare to create your transition
- You might also try adding a short shape player or a solid with additional effects and masking to enhance this overlap transition
- Annamae The opacity and the effects to complete the transition
- Render the shop and put it in your nonlinear editor to add to your movie making sure you use a suitable production quality (that means no MPEG or MP4's) video format
That is the procedure to almost all visual effects. It is easier to do if you have a clean plate to put below your actors so you can make them disappear by simply animating opacuty. You isolate only the frames that require additional processing like the extra blur and light flash, then you complete the shot. It's just layers and effects. If your flash transition is going to take 20 frames them those 20 frames are the only ones you should be applying a text to. The actual settings you will use depend on the look you want to have the footage you have.
If you do not know how to rotoscope or mask then type Roto, or mask, or whatever effect you don't know how to use in the search help field at the top right-hand corner of after effects and you will be directed to Community resources that will help you figure out how to use the tools