Assuming all the content objectives, etc have been refined...
We have revised our processes several times. Currently what works best for us is this: I have the content expert develop a rich outline of what they want to cover in a learning module. I use to have them actually write a bunch of script when they did the outline. But, I found that too often it was a long, wordy document. So, instead I have them just do expanded outlines with some notes and thoughts about what needs to be covered.
Once we have that complete, we have a production meeting where we actually convert this outline into a "broadcast" script, complete with transitional statements, visual cues for the narrator, and other things which make it sound like it's conversational. Most of the content experts I work with are good teachers, but they write different than they would present it if they were standing in front of a group of people. I ask them to actually close their eyes and visualize they are saying this to a group of people. "How would they say this to someone?"
This has transformed our product. It's now much more conversational. The narrator sounds more natural and the end result is so much better. Also, in that production meeting I often have the narrator there too. They are usually good writers and can really help convey the ideas.