Since Anne gives us an account of the daily interaction of the men and women of the annex, there is no verbatim re-telling of any dialogue, per se. What we do get, however, is a summary of how the interaction goes, the effects of what the discussion brings with it, and how it will affect the rest of the people who are also in hiding and living through it all.
In the case of the Van Daans, we know a few things from the direct and indirect characterization that Anne gives the husband and wife dyad. Of Mrs. Van Daan, Anne has absolutely nothing positive to say. In fact, Anne is so done with her that she even describes the woman the following way:
...she is very moody and has been removing more and more of her belongings and locking them up. It’s too bad Mother doesn’t repay every van Daan "disappearing act" with a Frank "disappearing act."
During one particular fight between the Van Daans, who develop slowly into very greedy individuals, we learn that they are broke and that this is a huge deal for Ms. Van Daan, especially, who has always wanted to live keeping up appearances. For this particular reason, Mr. Van Daan wanted to sell Mrs. Van D's rabbit fur coat. After finding a furrier, Mr. Van Daan received 325 guilders for it, which was a lot of money.
Mrs. Van D, who was still clueless about her reality, expected to save that money to buy a new wardrobe after the war was over. When Mr. Van Daan tried to make her see how much they needed the money for other things, the situation in the house becomes unbearable.
We know that Mrs. Van D is impertinent, stubborn, and does not shine at all in adversity. Conversely, Mr. Van Daan is a put-upon, feeble, and easy to manipulate man. Both personalities together are a recipe for disaster. In times of turmoil, their interaction becomes even more strained and more stressed. When this turmoil entails the loss of their lives and everyday freedoms, the situation worsens even more, still. Just think about losing the very things that make the fabric of what you are made of. It is the stuff that makes nightmares come true.
This is why, in more than enough occasions, the Van Daans get in fights and discussions that bring down everyone.
It took some doing before Mr. Van D. Could make her understand that it was desperately needed to cover household expenses. You can't imagine the screaming, shouting, stamping of feet and swearing that went on.
It is clear that their emotional instability is a reflection of the instability of everyone's psyche at that point. Living crammed up in an annex, with people not of your choosing, and questioning everything that you know, cannot be something that entices mental health.
Therefore, Anne shows their interaction, the reactions that they cause, and the influence of their negativity as the focal point to describe exactly how their relationship operates.