Capitalize a sound, prop, or action only when it has to jump off the page, and a speaking character's name at first appearance, so capitals still mean something.
Direct answer
Capitalizing a key sound in action — a door that SLAMS, a phone that RINGS — is a common way to make it land, and many writers do the same for a prop the story turns on. It is emphasis, not a requirement: use it sparingly, because a page full of capitals stops signaling anything. Separately, a speaking character's name is conventionally capitalized the first time they appear.
On the page
INT. ARCHIVE - NIGHT
Mara lifts the blue box. Something inside RATTLES.
Down the corridor, a door SLAMS.
ELI (40s, rain-soaked, out of breath) fills the doorway.
ELI
Put it down.
Evidence weight
Final Draft's guidance names key sound effects, props, and actions as fair game for capitals and warns that overuse slows the read and dilutes the emphasis. A reader skimming only the capitals on a page should get its most important sounds and objects.
Some writers see capitalized sounds as a holdover from shooting scripts and leave them out of a spec; others use them as a rhythm tool. Neither choice is wrong. What reads as careless is inconsistency — capitals that appear for trivial sounds and vanish for important ones.
Not one rigid template
Capitalize the sound word itself — SLAMS, RATTLES — when the audience must register it: a clue, a threat, or the cue for what happens next.
A speaking character's name in capitals at first appearance is one of the most consistent feature-script conventions. After that the name returns to normal case in action.
Some writers capitalize more freely to drive the pace of an action-heavy scene, while others leave sound capitals out entirely. Both are current practice; what matters is that the choice reads as deliberate.
The door BURSTS open. Glass EXPLODES across the floor.
Mara DIVES behind the shelves as the alarm WAILS.
Reader and handoff friction
Choose for the recipient
In a spec, capitals should make the read faster, not louder. Capitalize the few sounds and objects that turn a scene and let the rest of the action carry itself.
Capitalized sounds and props can help departments find what a scene needs, but the production's own breakdown decides what is pulled. Keep emphasis consistent once pages circulate so revised pages match the ones already in use.
In Plot, capitals in Action are yours to place: type SLAMS or RATTLES wherever a sound has to land, and a new character's name in capitals at first appearance. Scene headings, character cues, and transitions go to capitals on their own as you write and in the PDF, so the only capitals in your action lines are the ones you chose.
Asked next
No. Capitalizing a key sound is a common convention for emphasis, not a formatting rule. Use it for the few sounds the audience must register and leave ordinary sounds in normal case.
It flags a new speaking character for the reader and for casting. After that first introduction, the name returns to normal case in action; the character cue above dialogue is always in capitals.
Only when a prop matters to the story — the weapon, the letter, the key. Shooting scripts sometimes capitalize more for the props department, but a spec reads better when capitals are saved for what turns a scene.
Usually sound effects. On a film crew it often means special effects instead: practical work on set, like rain, fire, or breakaway glass, as opposed to visual effects added later. Radio and audio-drama scripts start a line with SFX:, but a screenplay doesn't. Write the sound into the action, and capitalize it only when it has to land.
Action
The descriptive prose of a screenplay — what the audience sees and hears, written in present tense. Also called description.
Diegetic sound
Diegetic means part of the story's world. Diegetic sound is sound the characters hear, like dialogue or a car radio. The score is non-diegetic: only we hear it.
Character introduction
The first time a speaking (or important) character appears on the page: name in caps, plus a short visual or behavioral hook that makes them legible before they talk.
Script breakdown
A script breakdown is the process of going through a screenplay scene by scene and tagging everything production needs, such as cast, extras, props, wardrobe, locations, vehicles and effects, so the shoot can be scheduled and budgeted.
Browse the screenplay formatting reference
Rendered examples and accepted variations for common screenplay situations
Inserts, screens & written material
Put the exact words the audience must read in action, and use an INSERT only when the change in visual emphasis is itself important.
Pre-laps & sound bridges
When the next scene's sound arrives before its picture, write it just above the scene it belongs to: a (PRE-LAP) cue for a line of dialogue, a PRE-LAP line in action for a sound.
V.O. vs O.S.
O.S. means a speaker is in the scene but outside the frame; V.O. covers narration and voices outside its physical space. See screenplay examples of both.
