You'll also want to suspend something real on set to represent the cg characters eyes for correct eyelines. As simple as pitting a tennis ball in the end of a stand or as complicated asa stand-in actor or even mocap - gollum, and everyones favorite, Jar-Jar Binks - put something in there.
If the camera isn't going to be locked make sure you have adquate clear clean track reference points, preferrably ones that stay in frame and are unonstructed for the entire duration of the shot. But don't make them difficult to paint out by using large markers or LEDs that cast interactive light.
Be very careful to block out your shot as far as where ther cg character will go and what it interacts with, and what passes infront and behind it.
make careful and complete notes of camera position, distance, height, lens info, light positions and types etc. You'll need these for your cg camera and for tracking. Shoot reference photos to help.
shoot a clean plate if you can.
When it comes time to comp, hopefully your cg render will be pretty close to how it needs to look to be believeable. Make sure any tracking is NAILED- no slipping! Remember to include details like matching film grain and blending edges...don't forget to premultiply to get rid of black edges.
Hope this helps,
jon a