I usually try not to go inside-out but outside-in. For me, this means, I first try to imagine the broader context of things (the background) which then gradually leads me down an association chain that gets increasingly more detailed the closer we get to the actual scenes (the foreground).
Neil Gaiman once described a writer as a painter. A painting will always need a foreground and a background. The foreground needs to be detailed, but the background just needs to pretend to be there, so the players get the feeling that there would be something there, if they went and check it out. So if you spend a little bit of time on first painting a rough background, it becomes way easier to paint the foreground, too.
In a recent campaign (loosely based on the pretty good
BBC series "Taboo") I decided on a generally rather dark and dirty tone, which automatically created a plethora of details in my head I could use to generate more authentic descriptions.
The main theme was: Brutal, dirty revenge, so of the top of my head, a beautiful summer-day wouldn't fit as a background for my my narrative, so it had to be dark, cold and rainy
- Therefor, the perfect time of the year had to be late fall. This already creates lots of details
- It's often cold, wet, cloudy and dark
- There is mud everywhere, carriages get stuck more often
- There are more beggars on the streets and in the city, as it's getting colder
So now that we have the outside, we can go further inside to visit the foreground of the "painting", e.g. the room of an npc:
As we're talking about a cold and depressing mood in the back, it'll be very likely cold and depressing inside his room, too:
- a cold hearth (warm would be way to comfortable)
- water dripping into a pot in the corner (it's constantly raining outside)
- dirty windows (let's make it even darker)
- mold on the walls (of course there is no proper insulation)
- the mold of course smells bad
- a constant draft (remember the bad insulation?)
- lot's of empty wine bottles (a guy living in this room does have to compensate for a lot)
- dry or stale food
- Old insignia and rusted medals from better times (a guy with a proper job wouldn't be living there)
- His dirty, smelly clothes
- His mud-crusted boots
- And as a final touch we'll go very deep inside, now we're in the npcs head: pictures of his family that is definitely not living there anymore (doesn't have to be plot relevant, but that's really depressing)
Once you're at this point, you've already painted a picture of the outside and inside that is strong enough to allow the PCs to interpolate the gaps between the descriptions. They'll already have a strong conception of what kind of guy lives there, even before they meet him in person.
Of course he'll look tired, have bloodshot eyes, smell of liquore and won't be properly shaved, of course he'll be sad and tired, as he misses his family.
Finally, something really nasty and amazing you can do every now and then is to mess with players heads by creating this kind of picture and expectation and then ripping it away from under them in an instant, creating this beautiful dissonance that leads to great inspiration and role-playing.
What if the guy comes home and is actually very well healthy, groomed, polite and nice-looking?