To put a character from a shot into another shot is tricky but doable, sometimes.
If the two shots are steady, you should not have much trouble?
If they are in motion then:
- Track the camera movements (including rotation and zoom) from shot A (the one you want to add a new character in) and link a Null object to it.
- Put shot B (the one with the new character) above shot A with transparancy.
- Link shot B to the Null object with the movement of shot A
- Thanks to transparency, look if your character from shot B looks all right in shot A (good angle etc) and place it at the right place and with the right scale.
- Inside shot B: make a mask (rotoscoping) around the character and animate it along the shot. Don't esitate to make several simple masks (one for each arms for example) rather than ONE complex shape.
- Once your character is "rotoscoped" you should see him moving in shot A as if it was filmed. Maybe add soft edges around him to make him looks less "cop py paste".
- Check the motion blur case (Also don't esitate to animate the softness of the edges when the characters is moving fast: somtime it works all right to simulate motionblur and it's faster to render.)
- Make color corrections, maybe add grain. etc.