Come out from behind the plate, out of the circle, in fair territory and observe. I will usually watch BR round and touch first and look for any possible OBS. This is not my responsibility, but in the event of a gross miss by my partner from not watching, I may be able to help if needed. If the throw comes into 2nd, I observe. If the throw goes back into 1st, I move towards the 1st base line for a possible overthrow. If the throw goes to 3rd, I move towards that line for the possible overthrow. If the BR commits to 3rd base, I will move to point of plate in order to get overthrow responsibilities and be ready to adjust if runner comes home. As the play develops, I also look out for possible rundown situations between 1st and 2nd, 2nd and 3rd. If I read that this is a possibility, I can start to move that way in the event it does develop. Even though BU has runner into 3rd, you are still umpiring.