Alright serious answer...
Whenever I've worked with a client to improve their casting, that person has had as much pleasure from the day as any skilled caster who caught lots of fish. In other words the angler takes home a much bigger prize than the fish when he/she gains a skill for life. Even if you don't make a quantum leap you'll never be faulted for trying, better yet if they have a breakthrough which does happen often enough.
As a guide I ask the client if he would like to spend a little time getting the cast down before moving to the next pool, or during lunch. Choose some transitional time when it's more appropriate. You'll know if they are unreceptive without much probing. If amenable, working together for a while improves the camaraderie and usually improves the number of hookups too.
Guiding is about offering a good experience, and most of the time it involves fish too.