For both reading and writing, I love details that allude to or recount the history of the relationship, be it from canon and fanon. I love stuff like Reigen's hand grasping Mob's with the desperate intensity he felt in the eye of that storm, or Mob watching as Reigen sips his morning tea out a mug Mob had personally handcrafted himself that Reigen refuses to part ways with, even with the increasing number of cracks fissuring throughout--two examples I totally made up off the top of my head as one that is rooted in canon and the other rooted in fanon. I love getting little stories out of stories and things that show the level of closeness between the characters, especially seemingly incosequential details.