When you’re putting together an adventure or detailing your next session, do you prep specific NPC dialogue?

Personally, I never have. I prefer to adlib NPC dialogue, which (like all adlibbing) produces a mix of gems, good lines, forgettable lines and, unfortunately, crap. Keeping track of prepped lines would be tough, and I think I’d be likely to flub them or fumble for them during play.

One of my group’s current GMs preps NPC dialogue for specific situations and encounters (notably, in our Stargate game’s recent — and excellent — time-loop adventure), and this approach seems to work very well for him. As a player in his game, it certainly turns out well on my side of the screen.

As someone who doesn’t do this, I’m curious to hear from GMs who do prep dialogue. How do you make prepped lines work in play? How do you spot situations where having lines ready in advance will be useful?

And if you don’t prep NPC dialogue, why not? Do you have the same concerns as I do, or is there a different aspect of your GMing style that comes into play?