I'm not sure I understand the question, so I've been hoping someone else would be brave and take a stab at it. Do you mean "write in the moment" meaning the experience of writing "in the zone" or with "flow"? Or do you mean "in the moment" in some other sense relative to the passage of time in the screenplay; e.g., do we see the events unfold in our minds' eye as we write?

Yeah, I'm a bit confused about the question too, but... Maybe it's about writing the scene description "as it happens", as the character experiences it, rather than writing a flat blow-by-blow description. It's an experience, not a crime scene report. Or not? Help us out here, Scott! :)

Possibly we are talking about an extension of the present tense discussion from the previous day. As the scene unfolds, are you (me), the writer, sensing the development of action in real time, being in the frame as decisions are made by a character? My take is that there are possibly two things going on. The first is that the writing act needs to be a process of character and incident growing out of each other, as a character moves into and out of a scene, there is change taking place and the change is both a development of character (pole to pole) and a development of action. This is a process of being within the moment. The second is the notion that writing occurs from the gut. The thinking has been done in the planning stage, once the story is 'nutted out' it is up to the senses to reveal the way the story is told and illustrated. To do this we have to 'let go' of the restrictions of thinking and trust our instincts to feel, taste, hear, smell and see what is happening as it happens. The point being, once you (me) the writer reaches the stage of writing, the better you (me) the writer knows the story, the more opportunity there is to be in the moment with the action as it unfolds. If you have to 'think' your way through a scene you run the risk of prejudging and losing its emotional power.