The guys that produce it are called "The MOB" Sky put up the finance for it and Terry has a big say in it all, as there is a budget to work to that in turn governs what book they can script for the film,thats one ofthe reasons they are doing GP next it was easier to make money wise.
If they had millions more in the pot it would give them a lot more scope.but being Sky they are not just catering to us Discworld fans but to the gereral viewers as they want bums on seats so to speak.
And the MOB don't want to disapoint the fans with an under par film.
He willnae tak' a drink! I think he's deid! , on the other hand though A Midgie in yir hand is worth twa up yir kilt.