I was thinking about Vladimir Propp's functions regarding role-playing games.
First it is needed to understand what Vladimir Propp discovered researching Russian folk tales. All of them belong to one type of tale, wich has 31 "functions". These functions have always the same order (except threesomes and repetitions etc.), but every tale has different functions in it.
These functions are nothing else, than deeds of the characters of a tale. As Propp discovered, the deeds remain the same, although the characters are always changing. For example the character that gives the hero a companion and/or a gift after testing him, can be a protagonist (a test is needed to know if the hero is ready for the gift) or an antagonist (after defeating him the hero aquires his/her possession).
It is useful in folk tales, because these functions always remain in the same order. But my opinion is that it can be useful in RPGs, too. The order of the functions are almost random, but the number of functions is much smaller than in folk tales and/or they are very structured. I am talking about rules.
For example in D&D3 the rules describe the functions: combat, cast spell, use skill, use feat. (Combat and spellcasting incorporates some using of skills and feats, but there is a possibility to use them alone without casting or fighting.)
Thus, when you are writing an adventure, the role of every NPC is not else then to carry a function of the adventure. For example: writing an adventure you decide, that the party shall be attacked. After that you decide what NPCs shall attack them. These NPCs' only role is to attack the characters, nothing else.
As I mentioned in a former post, there is a drama game in every RPG that is not explicitly described in the rulebooks. Theoretically that has functions, too, wich I started to define, but in this blog I will call functions only the ones explicitly stated in the rulesbook.
Every adventure can be drawn as a maze leading from function to function, where the nodes are functions, and the players' decisions can choose wich way to go.
Certainly this disregards the drama-game, wich functions are much more fuzzy in this regard, because the writer of the adventure decides everything about them.