I will mention this is my first play-by-post, so if you think I've done something wrong PLEASE tell me and correct it. I'll edit any comment as needed.
<Snipped quote by EruditeAssassin>
I think in a D&D Play by Post scenario, you cannot control the NPC characters as the GM would know how they would react to it.
This is the method I used when I DM'd a campaign previously.