When you are in a tense or difficult conversation with someone they will tend to see and hear what they expect from you or the situation. This can lead to misunderstandings or disappointments which ignite the spark of conflict.
If you find yourself in a negotiation or discussion where either or both of you are likely to be nervous or tense, start by talking about your respective hopes and intentions for the outcome of the conversation you are about to have. Explain your position carefully and invite the other to do the same.
Purpose and intent
If you both understand the purpose of your eventual exchange, and the outcome you are hoping for, the difficult conversation will be more manageable. It might even turn out to be easier than you'd anticipated.
A difficult conversation handled well, will strengthen, rather than weaken a relationship. Even between convinced adversaries, it can help develop mutual respect and even trust.
I call this the 'meta-conversation'. It's a clarifying tactic often reveals unexpected information, about expectations or motives for example. In some cases this will really help your subsequent conversation, in others, it might mean that something else needs to happen before the main conversation can go ahead.