At the start of my scene the avatar's look at target is the book in their lap. The camera is orbiting the avatar from back to front to back again. When the camera reaches a point where it is in the eyeline of the avatar I end the book as the target and a few frames later tell the avatar to look at the camera.  My problem is that the "look at camera" affects the avatars head movement even in frames outside of when "look at camera" constraint is in effect. The avatar's head snaps back and forth as the camera moves as if the two "look at" constraints are interfering with each other even though they are never active at the same time?. 

Is there some setting external to the "look at" constraints that is causing this, or some way to fix the problem?

Thank you for your excellent demonstration on how to manage the look at feature. Your video was just what I needed to see what I had been missing.
Thanks

That is great! Glad it helped.