A talented boom-op will do very small adjustments from person to person. In the clip I posted, if you watch the mic closely, you will notice that the angle changes slightly as the speaker changes; he is holding the mic between the actors and gently twists the boom-pole to change the angle. The scene is actually a poor example as there are a lot of rapid-fire exchanges, a really tough job for a boom-op. BTW, a really good boom-op knows the script at least as well as the actors.
https://www.youtube.com/watch?v=CNp6sndYJLo