Here is a great article for anyone directing talent on-set. It is very easy to run your actors/spokespeople into the ground through retakes and poor communication if you are not careful. On more than one occasion, I have sat in the editing suite working on a project and have been shocked to realize just how many takes I actually ran on a particular shot. What may have seemed like about fifteen minutes of retakes while on-set was in reality close to forty-five--and I wondered why my talent was getting a little frustrated! It is certainly important that you leave the set with usable material, but you should be competent enough to realize when you have something you can work with. This article from www.nofilmschool.com expands on this idea a little and will be helpful to you, I believe.