I have been watching Zach King videos on YouTube (the Vine magic guy) and I'm curious how to edit two shots together so that body movements stay the same, yet an element is brought into the shot. I'm ot even sure what the name for this is.
An example is in the link below. In the first two Vine videos starting at 3:03, he clearly meshes two shots together somehow but the placement of his body stays consistent. Immediately after, he makes it appear as if he pulled a cat out of his computer screen (again, obviously two shots) but his body movements stay consistent.
Usually you need three shots. The easiest thing to do is lock off the camera and shoot a clean plate. A clean plate is a shot without the actor in the shot. Then you should your first pass with the actor, then the second pass.
In post you put the clean plate on the bottom, the second pass next, then the first pass. The trick comes when you edit between the first and second pass. The first pass with the actor is rotoscoped to remove the actor from the background. The clean plate helps fill holes, and the second pass reveals the effect.
if you want to use a handheld or moving camera and you also need to learn how to do motion tracking, stabilizing, and retiming. Things get a lot more complicated very quickly and require a lot more preproduction planning.