3. Impressions

At this point, you should have a solid concept for the NPC, enough that you can start portraying them.

If you're creating this NPC on the fly, frame a scene with them (if you haven't already) and give up to 3 impressions about them, their surroundings, or what it's like to be around them.

If you're prepping the NPC in advance, decide on these impressions now and write them down, to whatever level of detail you find helpful. Bring them up when you first put the NPC on screen.