I've only gotten this to work in one way (though that might be lack of imagination on my part).
I placed the NPC in the middle of a room and put a very large marker around them. When the player steps into the room, and breaches the edge of the marker, they get a pop window with dialogue in it (threats or explanation of evil plans, etc). Hopefully the radius of the marker is large enough that you don't begin combat right away. That requires some trial and error.
The biggest downside is that you can't get up close to the NPC that's talking with you. I'm sure there may be a way to do this with a single NPC, but I've also only done it with groups.