I would setup a background image. By setting the axis to camera you will always see the image or movie while looking in camera view Numpad-0. If you select a movie it will change as you change the current frame in blender allowing you to follow the animation.
You will most likely want to ensure that the render settings match the original video so that there is no distortion or misplacement. You can also choose whether the image shows up in front of or behind your models.
This is a viewport only display, the image will not be included in the final render, you will need to composite it over the video.