Keeping in mind the schedule you developed in the planning stage, your video producer will have a set of sign-off milestones with approximate dates. Delays in achieving these milestones can pretty much guarantee delays in final delivery.
Since making a video is a process, certain things have to be done in order. You can’t decide to make an omelet after you’ve already fried your eggs!
At each stage, you will be required to approve the work before your producer will continue to the next step. This is basic corporate CYA, but it covers yours, too.
Any change or extra work after approval will necessarily require additional charges. If you need time to consult colleagues - take it. That is better than having to revise completed work, blow your budget, and your deadline. If you are unsure, or wonder at different options, ask your video producer to explain their rationale, and offer alternatives.
It is very important to the relationship, however busy you are, that you take the time to read the script, follow the link, review the edit, etc., so your producer knows that the project is your priority. It takes days or weeks to make, but only minutes to comment. Same-day feedback is always appreciated, and ensures your video stays on track.