The client's line was that one clip should work everywhere. We delivered widescreen, cropped it square, then found the product had been sitting in the left third the whole time and the vertical version was a shot of an empty wall. Ratio is not a delivery setting. It is part of the composition.
Vertical fills the screen in short form feeds and story placements. Composition is stacked rather than spread: subject high or central, text in the lower third, movement mostly toward or away from the viewer.
Widescreen suits landing pages, embedded players, and anything with a horizon. It rewards lateral movement, a pan, a tracking shot, two subjects in conversation.
Square is the flexible middle. It survives a grid, a carousel, or a marketplace listing, and it crops least badly into the other two.
Pick the format the campaign will be judged in, usually the paid placement, and compose for that. Then decide the fallback while you are still in the brief. A composition with margin survives a crop. A tight one does not.
One habit covers most of it: keep the subject inside the central square of a widescreen frame. That alone makes vertical and square versions of the same shot possible without regenerating anything.
A push in reads well in vertical because it adds depth to a narrow frame. A lateral pan reads badly there, because it has nowhere to go. Widescreen is the reverse. If the ratio is decided, the move is mostly decided too, which is why ratio belongs in the brief rather than the export dialog.
None of this is exotic. It is the discipline of shooting with a delivery spec taped to the monitor, moved one step earlier. With generated footage you can plan the format before a single frame exists, and that is a genuine advantage over a shoot day where the ratio gets decided in the edit suite after the budget is spent.
Tools that let you pick the ratio at the prompt stage make that discipline practical rather than theoretical: cinematic AI video generator.