It's interesting how different writers handle this, honestly. There's definitely a pattern to what gets written, but which characters get the spotlight and how they react says a lot about the author's favorite dynamics.
A lot of people immediately jump to Subaru's breakdowns and the aftermath of each loop. I've seen countless fics where Emilia or Beatrice finds him after a particularly bad death and he just completely shatters, or he gets weirdly quiet and they have to pry the trauma out of him. The 'comfort' part of the hurt/comfort equation is massive here. Emilia usually goes into a protective, almost desperate caretaker mode, trying to shoulder everything for him even when she doesn't understand it. Beatrice's reactions are more interesting to me—she's less about overt emotion and more about a quiet, fierce loyalty, maybe finally forcing him to rest or using her library to search for answers he won't give.
But the real variety comes from the less obvious perspectives. Otto is a great example. A good Otto-centric reaction fic doesn't just have him being shocked; it has him being shrewd. He'd piece things together faster than anyone, connect Subaru's knowledge to his strange moments of foresight, and then have this whole internal crisis about friendship versus the sheer terror of what his friend is enduring. I've read one where he quietly starts keeping a journal of Subaru's 'bad days' to look for patterns, which felt so perfectly in character. Meanwhile, characters like Garfiel or Frederica have these explosive, protective reactions—they want to fight the unseen enemy, but there's no enemy to swing at, just Subaru's internal scars.
Honestly, the fics that bore me a little are the ones where everyone just cries in a big group hug. The messy ones are better, where reactions clash. Like Rem, post-memory loss, being confused and jealous of Emilia's closeness to Subaru, while Puck is detached and analytical, and Roswaal just watches with that unsettling smile, already three steps ahead. That tension is what makes the concept work.