Just in case some reader here thinks it odd to create on the fly as Rob and I usuall are want to do:
The main difference between formal creation of material and doing it as one serves in the role of GM is spontenaity, that allowing the material created on the spot to beter suit the player group ad the situation at hand. Otherwise one must set forth the material to be played and recite it more or less verbatim, forcing the group to its mold. the creative demand is muc the same, but the free-style method usually allows fo more enjoyment b all participants.
I recommend it to all GMs able to manage such playing style
Cheerio,
Gary