The last post was getting too long, so I split off some of my random notes and put them here.
First, whenever possible, speak in character. You probably tell your PCs this all the time. But think about the following sets of responses:
* He says he does not know about the relic of Temerin.
* "Don't hear a lot about relics in my line of work."
* "The relic of Temerin? It sounds valuable. Is it?"
* "I'd have to ask Master Rei."
The last three are all more dynamic and interesting. They are more fun to play and listen to. Speak in the NPC's voice whenever possible.
Second, try to keep the number of NPCs on stage at one time limited. No more than two, whenever possible. If you need to bring everyone together, such as for the PCs to unmask the murderer at the end of their investigation, treat it like the end of Murder She Wrote. Sit everyone down quietly and let the PCs call NPCs out, so to speak.
If you have to have more than one NPC on stage, NPCs from directing questions at each other, or in any other way running around the PCs. They are important, but talking to yourself is not entertaining. Let the PCs cut a word in edgewise.
If a PC is out of the room when a new NPC is introduced, use that opportunity to re-establish the NPC later. Have them meet the missing PC again; maybe use the same trick or joke that they pulled on the other PCs. You want NPCs to have as many opportunities to define themselves, so if they have an interesting introduction, milk it if you get a second opportunity.
Finally, let the PCs win insult-offs. For some reason, NPCs become more memorable after the PCs have humiliated them.
This is because PCs are jerks.