So you have to write a bit of dialogue. Where to start?
With the first draft, dialogue doesn't need to be subtle. Write on the nose. They say what they think regardless of consequences. Enjoy! This part can be fun. Just sit back and watch as the fireworks explode.
So what to do on the second draft? Two things. Watch what they do when they talk. The conversation isn't just about what they say; it is also about that they do. So pay attention. What do they do? Turn their back, sit down, squint?
Next, work out what they are hiding. Characters will always hide their true meaning behind their words. The subtext is important.
Learn from the masters. Watch a politician evading questions. Politicians can never be seen to be losing. Someone else is always to blame. They believe themselves to be the masters of deception. As if!