I'm wondering if DeForrest Kelly had a clause in his contract that required him to appear on screen a certain number of times. How else can you explain the fact that on several occasions, Dr. McCoy just wanders onto the bridge, looks around and then leaves. Frequently without saying a word. Its odd.
One cannot help but notice that the one time there is actually a medical situation on the bridge, its Dr. Chapel that does the treatment. McCoy just seems bored for most of the film.