I deal with similar issues.
I receive scripting for our front line staff to use during customer interactions, and many times the first thing I ask is the same as Steve: Is this verbatim, or can I tweak it?
Imagine giving an entry-level customer service agent a suggested script riddled with executive-speak and uncommon, polysyllabic words, like "polysyllabic."
One issue is the executive-speak, but the other is Andy's observation - writing for reading is much different than writing for speaking.
Thankfully, I've built a reputation and relationship with my stakeholders where they usually allow me to make the scripts more "speakable."
To answer your original question, Andy, I create training both for incumbents and new hires. The incumbents I obviously expect to have at least a baseline knowledge of the topic, but for the new hires I need to explain things from scatch.
And while I have enjoyed writing as a hobby since high school (and as a training/elearning professional for the past 15 years), by no means do I consider myself a professional scriptwriter. But another hobby of mine is multimedia production and voice work (I do my own narrations), so I know firsthand the differences between writing for reading versus speaking. This awareness has been helpful for me when writing my scripts.
But for someone that doesn't have the same experience, I think the best advice is similar to what Allison stated: Even if you're not going to narrate the script yourself, read it to yourself out loud, preferrably recording it and then listening to the playback. Only then will you be able to hear how your writing actually sounds.