The number of stories that pit Reagan and Brett directly against each other is surprisingly small, in my observation. Most writers seem to default to them being a united front against the chaos of the rest of the office or some external corporate threat. Which, honestly, feels a bit like a missed opportunity.
When conflict does pop up, it often stems from their core personalities clashing. Brett's relentless optimism and naivety can grind against Reagan's cynicism and hyper-competence. I've seen a few where Brett's desire to connect on a personal, emotional level frustrates Reagan, who sees it as a distraction from the mission's efficiency. The resolution usually isn't a huge blow-up fight; it's more about one of them, typically Reagan, having a quiet realization. She'll notice how Brett's method actually solved something her logic couldn't, or she'll reluctantly admit she values his presence beyond his utility.
It's a very low-stakes, character-driven kind of drama. The tension comes from whether their friendship can accommodate their differences, not from whether it will survive a betrayal. That's probably why I gravitate towards these quieter fics more than the big action-packed crossovers—they feel true to the show's vibe of these two weirdos figuring each other out.