I think as a writer one of the more challenging things to learn is to make your characters uncomfortable. There's a pretty simple reason why it's difficult: we don't like to be uncomfortable ourselves. And so within us there are these taboo things that we would never say or do. Like we'd never start asking friends we're just meeting or haven't seen for twenty years about their sex lives, or pry into why they don't have children. Or we wouldn't keep asking our kids or friends what they could have done to make their wife want to divorce them.
But conflict is the engine that makes a good story run. And discomfort is a fantastic form of conflict. Because it can be really hard for people to address directly. Think of Cameron and Daphne with Harper: they keep saying and doing things that are clearly crossing the line with her—particularly Cameron. In 201 he's taking off his pants in front of her; in 202 he's grabbing her underwater and saying she thinks he's a materialistic pig. And of course she does (and he is); but saying that out of the blue is just not something that people do.
These kind of moments keep putting Harper in the position of having to choose what to do. And part of what makes them so rich is that every choice she can make will really only lead to more conflict and discomfort. Go along and be trapped having to deal with Cameron and Daphne make more uncomfortable moves; or call them out and deal both with the fact that you're still stuck on this vacation with them and her husband's potential frustration with her for doing so.
Bert's refusal to just let the topic of Dominic's infidelity go functions similarly. And not only does Bert keep bringing it up in really unexpected ways, he's willing to do so bringing up other generally taboo topics, specifically rape. That scene is fascinating, actually, in that he keeps saying the word "rape." And it turns out that alone is uncomfortable. He's not even talking about a real situation, just this horrendous Greek myth. But it doesn't matter. The idea and the word are enough to make everything very uncomfortable.
Although it's interesting, in Bert's case it's a bit of an open field as to whom he is actually making uncomfortable. Dominic certainly doesn't like it, but unlike Harper he also keeps saying so, pushing back.
From a writing perspective, I think the deeper intended victim is Albie, the family "peacemaker." Each time Bert brings his father's infidelity up and attacks his wife, it challenges Albie's capacity to stay in that role. How long will he accept this kind of treatment of his mom, and also his father's serial infidelity? Bert seems designed to keep poking away at his "good guy" vibe until he completely loses it. That's part of the joy of using discomfort as a form of conflict; it creates a sense of slow build. It becomes a means of creating anticipation.
The tried and true line of screenwriting is that you want to chase your main character up a tree and then throw rocks at them. That can be hard to do; as writers we can care so much about our main characters that we instinctively try to protect them. But part of Mike White's genius is that he clearly asks, What is the thing that these characters would find the most uncomfortable?, and then forces them into exactly that.
In our own scripts we might ask ourselves that question: What is the thing my protagonist would find the hardest to deal with? And am I currently forcing them to deal with that? Or is there more that I could do?
