Usually, in a book, they don’t specify what the perspective of this retrospective narrator is. We just accept that, as some point after these events, they sat down to tell us this story, even in books like “Double Indemnity”, where the hero keeps narrating in the past tense right up until the moment he dies in the final line.
Some book narrators say things like “I didn’t know then that that would be the last time I ever saw him”, but more often they don’t acknowledge that they now know what happens after the events they’re describing. Nevertheless, we have the sense that they have a little more perspective than someone would have if they were actually experiencing these things in real time. At the very least, they know which things to describe and which to leave out, or else they wouldn’t be able to tell the story. They can’t just indiscriminately describe everything they see and hear.
But the reason that screenwriters write in present tense is to remind themselves that their heroes lack that perspective. This stuff hasn’t already happened, it’s happening now. In a movie, the audience actually sees and hears the same things hero does, not just what the hero chooses to describe.
This has advantages and disadvantages for the moviemaker. Movies have more ability to drop clues that the hero and the audience miss, buried amongst those sights and sounds that aren’t important to the story. Books heroes have to line up all their perceptions in a row, one after another, and can’t turn the volume down on a key clue so that it’s low in the mix. Every reader will see every word, and they will know that each word is important to the story. We don’t really buy it when a first-person narrator misses a clue, because they were the ones who told us about the clue earlier, so they obviously spotted it.
I talked yesterday about how audiences demand that movie heroes have faster reaction time than book heroes. The corollary of this is that movie heroes have to act with less information and less perspective. Book heroes have already processed their story. Movie heroes are attempting to process everything at the same time we are. They’re living in the present, which makes them a lot more tense.