In both pre-plotted adventures and more story-telling ones, a lot of situations occur where monsters/npcs/magical items/geography appear, which wasn't thought of beforehand. I.e. I do not have the stats for them available.
How do you come up with these items/characters/details on the fly? Is it all just improvisation? They're obviously improvised, but what I am asking is; do these follow other gaming rules. For instance, a rule of "yes, but..", rather than the system combat rules. Since the cat following the party just revealed itself to be Tankors long-lost twin, a shape-shifting druid, I don't know what spells he can do or what AC, so all kind of rolls will be without values to check against. Is it ok to let the story decide rather than the dice?
...or.. Do you adapt items from books, monsters manuals, etc. Is it possible to pre-make lots of npcs and just pick a sort-of fitting for the new character and then fill in some details (initially left blank). In that case all the newly introduced game elements would fit into the system and the dice could rule again?
Maybe there are some other ways of solving this on-the-fly as the two I outlined. I can imagine that being very experienced with a system the GM would know what AC and skills a 5-th level ranger or a little blue dragon would probably have.. but I'm sort of looking for advice for those of us who can't pull stats off the top of our head.