Your TV spot or online film will exist in a finite length. It’s not an experimental art project that will just stop when it’s done. It’s advertising. So come to grips with that fact upfront. Take a sheet of paper and divide it into thirty frames (or whatever the length). Noodle a picture, and a description, of each second of the spot. Write in the dialogue and time it. Don’t cheat. Make it fit now, make it clear now, make it great now, and your chances are much better that it will be great at the shoot and on the screen. Don’t leave it to the director. Don’t presume it will fit as a linear story, in real time. Plot it out then feel confident because you’re certain.