Short answer
For a finished AI video ad, write a brief, not a shot description. Use five lines: format and length, audience and goal, one message, facts you can support, and tone and structure. Budget about 2.5 spoken words per second, review the script before you render, and keep exact numbers, logos and on-screen text out of the visual description.
Two kinds of prompt
Clip generators such as Luma take a shot prompt: who is in frame, what moves, the camera and the light. Luma’s guide (August 2026) uses six parts (subject, action, setting, camera, lighting and style) and says to put the most important details in the first 20 to 30 words.
Tools that make a finished video from one idea take a brief: who it’s for, what it should say and how long it runs. HeyGen’s prompt guide (updated October 2026) says the same: open with duration and orientation, then name the audience and one message. Most agency work is the second kind, so this guide is about briefs.
The five-line brief
- Format and length. “30-second vertical video for Instagram Reels.”
- Audience and goal. “Commuters; get them to click Shop now.”
- One message. One point per video. HeyGen notes that several messages produce rushed scenes.
- Facts you can support. The product name, the price and two or three real features. Don’t let the tool invent claims.
- Tone and structure. “Practical, upbeat. Hook in the first 3 seconds, one proof point, call to action at the end.”
Budget the words. HeyGen suggests about 2.5 spoken words per second, so a 30-second ad holds roughly 75 words of voiceover.
Example 1: product demo
Make a 30-second vertical product demo for the Trailhead insulated bottle. Audience: commuters. Message: keeps drinks cold for a full workday. Features to show: leak-proof lid, fits car cup holders. Tone: practical and upbeat. Hook in the first 3 seconds. End with “Shop now”.
Example 2: UGC-style ad
Make a 20-second UGC-style vertical ad. A friendly person talks to camera about why they switched to the Trailhead bottle. One feature only: it fits their car cup holder. Casual language, no jargon. Start with a question. End with a call to action.
Example 3: explainer
Make a 45-second explainer for an invoicing app aimed at freelancers. Three scenes: the problem (chasing late payments), the fix (automatic reminders), the result (paid faster). Calm, clear voiceover and light background music.
The product and app names are made up.
Edit the script before you render
Review the script before you spend render time. In Fabeo, you describe the video, add the client’s product images and pick a format and length. Fabeo drafts the script scene by scene, and you can change the text or voiceover of any scene. Fixing a prompt at the script stage is cheap; every regeneration of rendered video uses credits for the seconds you re-render.
The same brief works as a chat message in Claude or ChatGPT: see the Claude guide or make a video with ChatGPT. To turn a client call into a brief first, see how to brief an AI video tool from a client call.
Mistakes that waste renders
- Vague mood words. HeyGen says generic terms like “cinematic” do little. Name concrete decisions instead.
- Readable text and logos in the visual prompt. Luma advises never asking a model to render readable text or logos; add them afterwards. HeyGen suggests keeping generated on-screen text to a word or two.
- Exact numbers. FlexClip warns that models struggle to keep numbers consistent. Put prices in the script you review, not in a visual description.
- Several camera moves in one clip. Luma suggests one move per clip.
- Stacked messages. One video, one message.
What it costs to test prompts
Fabeo’s plans are monthly (pricing):
- Free: $0, videos up to 30 seconds, with a Fabeo watermark.
- Professional: $12 for 4 rendered minutes.
- Enterprise: $24 for 8 rendered minutes.
- Agency: $39 for 15 rendered minutes, about $2.60 each.
Top-ups are $3 per rendered minute on every plan, and paid plans export without a watermark.
Keep a prompt library
Luma recommends storing tested prompts by brand, content type and platform, with the variable parts left to fill in. In Fabeo, a winning video can also become a reusable template: swap in a new product, brand or script and render again. See the AI video templates, or AI product videos for Shopify listings for product pages.
FAQ
How long should an AI video script prompt be?
Five short lines is enough for most ads. The voiceover itself should fit about 2.5 words per second of video.
Should I describe camera angles?
Only for clip generators. For a finished ad, describe the audience, message and structure, and let the tool plan the shots.
Why does the tool get prices or numbers wrong?
Models struggle with exact numbers in visuals. Keep prices in the script, where you can check them.
Next step
Try the five-line brief on a template, or see how Fabeo handles product videos and UGC-style ads.