character a asks "hey, will you pretend to be my date for a week for [excuse that could easily be solved without a fake relationship]?" character b, fully convinced of the futility of their DEEP AND UNREQUITED LOVE, figures this'll be a chance to spend time with them and possibly put their feelings to rest. character b is always wrong & it is always amazing.
having to SHARE A LIVING SPACE FOR ARBITRARY FIC REASONS. having to see eachother in their pajamas first thing in the morning, messy haired, drowsy eyed and soft faced. going from "you can have the bathroom first" to brushing their teeth beside eachother and feeling like this closeness has always existed (at the same time, painfully aware that it won't always).
related to the last one – ""practicing"" their casual touching so that it's easier when they're in public. feeling SWOOPS OF ARDOR AND AGONY when they feel the brush of a hand on their neck, or an arm loop around their waist. don't you love how fake marrieds/dates are always method actors who must FULLY INHABIT their roles. i love it. i live for it.
bed sharing. :^) we all pretend we're bigger than this but we are not.
"kiss me while everyone's looking."
the character who wasn't aware they were in love (maybe always had been) until the fake relationship is in full swing, realizing they have to sort their feelings out before their time together is up. sometimes they succeed and angst is minimal. most of the time they don't, really.
Telling eachother "i love you" in public and meaning it, heart aching over it, but bELIEVING THE OTHER TO BE ACTING.
The days leading up to the end of the arrangement where one of them, still confused and muddy about their Feelings and unsure how to break things off, stiffens to the casual, reflexive touching and puts their walls back up. the other one accepts and respects this as the end of their agreement and squashes back down all the hope they ever had, stuffs it next to the heartbreak they're ignoring deep in their chest.
when they realize they're actually fully and enthusiastically mutual about the way they feel and it's, like, two parts euphoria and one part agony because they just cannOT BELIEVE, the happiness tears them in two. maybe there are weepy or laughing kisses.