When sovereign states A and B are conducting some bilateral negotiation, they have a few options:
- A's ambassador can negotiate with B's government
- B's ambassador can negotiate with A's government
- Ambassadors or other representatives of both states can negotiate with each other in another forum, such as the UN or a third state
For example, in The Hunt for Red October, the Soviet ambassador to the United States negotiates with an American official in Washington intermittently over the course of the plot. What factors result in this pairing, instead of the American ambassador to the Soviet Union negotiating with Soviet officials in Moscow?
I have a better understanding of what factors would force the third option (for instance, a lack of bilateral relations between the states involved), but feel free to discuss them, too, especially if they help illuminate the other choices.