Start with a specific loss
A dangerous job is a situation; a character with something to lose gives it meaning. Write the objective in one line, then explain why failure matters to this person. You should be able to understand the stakes without a paragraph of backstory.
Choose one unanswered question for the final image. Adding a second twist usually takes time away from the first. The viewer needs to understand what changed before the trailer ends.
A fictional example you can adapt
A night courier is paid to deliver a sealed case before sunrise. A voice warns them not to stop. As they approach the destination, they recognise their own front door. The question is no longer whether they can make the delivery, but who sent them home.
For a twenty-second version, begin on the case lock, show the warning in a close shot, follow the courier toward the destination and end on recognition. The address reveal should be readable in the edit; do not rely on a fast, distorted generated street sign to carry the story.
Give every shot a usable opening moment
The opening image should show the instant before the action you want to animate. If a character needs to open a door, do not begin with the door already open. If the next shot is a reaction, it can start after the door has opened; the cut connects the two moments.
Create references for recurring people, props and places. When someone new must be clearly identifiable at a reveal, give that reveal a shot whose opening image already contains their approved appearance.
Write a sound column as well as a picture column
Use sound to focus attention: a lock clicking, an interrupted engine or a voice through a speaker. State the exact dialogue and who speaks it. Keep an off-screen caller off-screen; do not ask for an extra face merely because the voice is heard.
A music brief should describe the shape of the scene: a sparse opening, a build under the attempt, room around the important line and a final stop or sustained note. Check the native audio in each take before adding another layer of effects.
Choose twenty or thirty seconds honestly
Use twenty seconds when the premise and turn are simple. Choose thirty when recognition, a conversation or a change of location needs more room. A longer trailer should add clarity, not more unrelated events.
The thriller Closer Than You Think grew beyond its original twenty-second brief so arrival, discovery and betrayal could each be understood. Its published production is a useful example of reviewing the cut instead of forcing every beat into the first estimate.