Three things video prompts add
Beyond subject, setting, and style, a video prompt can specify motion, camera behavior, and duration. Each one tells the model something about how frames should differ from one another, which is information a still-image prompt never needs to provide.
Subject motion and camera motion are not the same instruction
Subject moves, camera fixed
- The dog crosses the frame while the background stays anchored
- Background objects keep their position and size
- Reads as a locked-off shot of a moving subject
Camera moves, subject still
- The dog stays centered while the background slides past
- Background objects change position and perspective
- Reads as a tracking or panning shot of a stationary subject
Same subject, different frame instructions
"A cat" gives the model a subject and nothing about time. "A cat sitting still while the camera slowly pushes in, three seconds" gives it a subject, a camera behavior, and a length. The second prompt constrains how the frames relate to each other, which is the information the model needs to keep the sequence coherent.
Duration is a constraint, not just a preference
Asking for a longer clip asks the model to hold consistency across more frames. Prompts describing one sustained action usually fare better than prompts describing several events in sequence.