After you have done all of your homework and really detailed who you are talking to and what that relationship is to you, then the work of staying in the moment begins.
Part of your job as an actor is to convince me that you are really listening and reacting to the other person or people in the scene. When you are in the moment, I can see you responding truthfully. You adjust your reactions to what you are seeing and hearing. If the opposite is true, and you find yourself getting stuck in your head; you may be focused on remembering the lines, worried about a sight line or the way your hair looks, then this distraction is visible. It prevents you from being present.
The truth of the moment is not just exhibited in your work as an actor, but also in your ability to assess and listen to others.