“Writing down the page” is an expression that basically means the thrust of the story moves down the page with action, description, dialogue and carefully chosen sparse elements, so the reader’s eye is continually pulled down the page and then to the next page. You should aim not to get the reader’s eye stuck in any one space on the page. Any block of text is to be avoided in this regard.
A cleanly written script usually takes me about 40 minutes to read cover to cover. If the script is written cleanly (i.e., with few block chunks of text on the page), scripts clip along well because I’m mainly focusing on dialogue with only character introductions and location establishing lines to pull me out of dialogue.
Conversely, a script that is unclear, confusing or convoluted with chunks of text might take as long as two hours to read. More often than not, these scripts take me between 75 to 90 minutes to read. But, this is annoying because a well-written script should only take 40 minutes to read. I thus know how poorly written something is by how long it takes me to get through it.
Please don’t forget that a reader might have 100 scripts to read at any given time, so the more you can do as a writer to help that reader better do his or her job, they’ll thank you. This includes writing down the page.
This is something I define as any element the reader needs to know that might get lost in a whole line of text. If I’m involved in a script but the writer’s prose isn’t magic for me (and sometimes it is but this isn’t the norm), then I’m reading primarily for story beats and major emotional beats. So, the more you can make a key element stand out visually on the page, the less likely the reader will be to miss that element. To this end, you may CAP certain words within a line or simply pull key words onto their own lines. This is especially effective within certain genres such as action, horror or thriller, where a specific physical element can take up its own space on the page and thus that beat will be pronounced for the reader (such as a crash, stabbing, shooting, etc.).
The reader wants to be able to have the flow move swiftly down the page, predominantly following the dialogue, and thus any chunks of text that appear to be overwritten could easily be skimmed. You do not need to write scenes wherein you separate every line of dialogue with an action line. This isn’t how most scripts are written and this is overwritten. Most screenplays introduce the scene with an action line at the top and then once the dialogue begins, that flows seamlessly. The dynamic and action of the scene should be felt within that dialogue.
Writing down the page doesn’t preclude you from implementing some carefully pruned blocks of prose. You can, and in some instances this is absolutely essential – for example, when introducing character. But, still, any chunks of text should be implemented wisely.
The more you can keep the reader’s eye moving fluidly down the page and then flowing onto the next page, the better off you’ll be.