1) ALL NPCs should have portraits, including non joinable NPCs.
2) All NPCs should have some vague semblance of a personality..
3) Unless it's a "special" (mentally challenged) NPC, they should have something to talk about, and interests beyond what the player wants to know.
4) The smaller the town the more bizzare the rumors..
(eg: there's been a huge fearsome beast lurking in the east forest!!
- actually it's trash collecting golem, made by a mage trhat's sick of littering villagers, or a group of kobolds getting their jolleys by scaring the villagers.)
Naturally the hero can boast that it was a greater beholder or some-such.
5) Converting alredy made NPCs is fine as long as you remove ANY references to "child of Baal" or any other bg2 plot references..