Anything that you can't improv easily, but not things that you can. Seriously.
Say that you make up names easily, but have trouble with motivations. Then you need something like, "Town Mayor -- wants to stay out of trouble, not have confrontations." Or even more details.
Yet, if it is the other way around, you might prefer, "Felipe de Diggerson, Town Mayor".
If you have trouble with both, you might combine the above listings. If you have trouble with neither, you might prefer, "Town Mayor" followed by a statblock.
Not having things you really need will slow you down and/or lead to laughable improv. Having a whole bunch of things you don't need will get in your way. This is one reason why there haven't been as many great sandbox products as you might think. Everyone has slightly different needs. It is tougher than it seems to organize such a product to keep from overwhelming people with a lot of stuff they don't need while they are looking for the parts they do need.