A screenplay is a blueprint, a technical document, that many people have to use. The producer to sell the film, the director to make the film, the actors to perform the film and the editor to piece the director’s footage into something sensible.
Every word on the page matters. If it can be removed, remove it. Adverbs, redundancies, repetitions, passive tense. Get rid of them all. The more white space on the page, the better. As a writer, don’t fall into the trap of thinking you can show off your fancy prose chops. If you want to do that, write a novel.
When writing action, think in terms of shots. One shot per paragraph.
Don’t use unnecessary crap like, “We see...” or “We hear…”. Keep it simple. Just describe what is happening. Also, avoid using technical jargon like “Medium-close up on...”. That’s the director’s, the cinematographer’s and, later, the editor’s job to decide on shots. You can influence this with the one line per shot rule, as more often than not the description and context of the shot will be obvious.
Simply describe what is happening in each shot. Keep descriptions in the active, present simple tense. There should be very little ‘is - ing’. ‘Paul walks towards the shop,’ is quicker to type and has more impact than ‘Paul is walking towards the shop.’
Only use a parenthetical (wryly) if absolutely necessary. Actors like to have a bit of leeway to choose how to deliver a line. Actors will, I believe, always inhabit a character far more fully than any screenwriter. They will also pick on subconscious cues the writer puts in that he or she didn’t even realise were there. I’m always in awe of the actors and the choices an actor makes. They are never quite, and often very different to, how I imagined it when I wrote it, and always a vast improvement.
Dialogue should be sparse. The old adage, “show, don’t tell”, will never die. If you can find a way for an actor to act a beat, rather than spout dialogue, go for that every time.
Always read your dialogue out loud. You’ll be amazed how shit it sometimes sounds, when in your mind it was oscar-worthy.
If you need to have Basil Exposition in the room, at least have him do something interesting while he’s doing his thing (this is a fun way to get subtext into the mix, where what the character is doing is contradictory to what they are saying). If nothing else, it’ll keep the audience interested while you bore them with facts they need to know.
Flashbacks, like everything mentioned above, should be used sparingly, if at all. If you need to establish a fact, find a fun and believable way to have it found out in the story’s present. A common trope, which I’ve seen a lot this year, is the ‘exciting open in the middle,’ where, following the advice of Something Startling Happens, the writers try to hook the audience in with a captivating sequence. All is well for ten minutes, and then suddenly the action stops, and they go, “Oh, hang on, I bet you’re wondering how we got here.” Then they go back to the original, boring beginning. Amsterdam was one of my favourite films of this year, but even that does it. They even had Christian Bale’s character say those words in the voice-over. I think Uncharted did it too, along with a few other films. It’s crap and lazy. Stop it! Nope almost did it, but just about got away with as the deranged monkey flashbacks were, in my opinion, really well done. They also dovetailed into the main story in a subtle way that took me a day or two to really appreciate.