We made a few videos 14 minute for math for the first year of the university. It was similar to stand-up, the speaker has a clear ideas of she was going to say, we took like 1 or 2 hours to pick the best examples and discus the path,. Anyway most of the details were improvised on the spot, with many sentences skipped or added or changed. We made a few takes of each 2-3 minute part, and glued the best.
Also we had to fix a lot of typos in the blackboard of the talk. I remember a "one plus zero is zero" that I had to fix. The hard part is finding a "one" that comes after a word that ends with an "s" because otherwise it sounds horrible.