Here are links to two interesting blog posts by Stewart Weltman in his "Lean and Mean Litigation Blog" on the subject of formulating a discovery plan before taking depositions. The knee-jerk response is depose everyone, but as this author makes crystal clear that is not always the smart thing to do.
A sample: "Rule No. 1 in developing a deposition program - you must have a clear conceptualization of what you want your case in chief to look like at trial and you also must anticipate what you believe the other side's case is going to look like. If you don't, then you are on a rudderless ship that will flounder aimlessly through discovery. How can you possibly know what to ask if you don't have a clue (1) what you want your case to look like at trial and (2) what you believe the other side's case is going to look like?"