One way of making a distinction between the screenwriter’s job and the director’s job is this: The writer’s job is to determine what to film, the director’s job is to determine how to shoot it. So when it comes to deciding what to include and what to cut from the script, it’s often a matter of choosing the level of detail.
Finding the right balance between too little and too much detail, is a large part of the art of screenwriting. Use too little detail, and you run the risk of not adequately getting your intention across. Too much detail, on the other hand, can feel restrictive and rigid. So the trick is to use just the appropriate amount of detail for the given beat, making clear what the beat is about while leaving enough room for the director to visualize the beat in his or her own way.
Take for example the famous scene from American Beauty, where Ricky (played by Wes Bentley) shows Jane (Thora Birch) his video recording of a plastic bag being blown about by the wind. Here’s how screenwriter Alan Ball describes Jane’s initial reaction to the video:
Jane sits on the bed. She watches Ricky’s WIDE-SCREEN TV, her brow furrowed, trying to figure out why this is beautiful.
Which is a lot of information for two short sentences. You know Jane is sitting on the bed, you know she’s watching the TV screen and it’s clear she’s feeling confused because she likes Ricky and is trying hard to understand him, so she can decide whether she really wants to get any closer to him.
This little description does a great job of pointing everyone on the set in the same direction. However, none of the information tries to force a particular way of shooting the beat. Which means that as the screenwriter, you hand the director (and the actors) all the necessary ingredients, but you leave it up to them to do the cooking.
A little further along in the same scene, while Ricky expresses the wonderment he felt when he first witnessed the flying bag, all Alan Ball says about Jane’s reaction is:
Now Jane is watching him.
Which is about as brief a description as you can imagine. It doesn’t give any concrete (visual) information other than: Jane has switched her attention from the video to Ricky. However concise though, this little snippet does have a clear function: it shows that Jane is more fascinated by Ricky than by the video. She’s not so much sharing his experience of the video, as feeling something similar while watching him. But again, it leaves the director and the actors free to portray this information in whatever way they find most appropriate.
Some directors will storyboard extensively before venturing onto the set, others like to “let it happen” and then compose the scene during editing. That’s all about execution—the how— rather than inventing the scene in the first place—the what— which is the screenwriter’s job.
As far as I’m concerned, the answer to my initial question is: Leave as much as possible up to the director, while making absolutely sure not to lose the essence of the beat.