Just a little ficlet initially posted to tumblr: see AO3 listing for link to the gifset it was inspired by. Originally posted (tumblr/ao3) 7/12/12
Sebastian had an occasional tendency of falling asleep in the bath. It wasn't a consistent trend, only happening after some of the longer, more arduous jobs, but regardless of his condition, he would not deviate from his acquired habit. Even exhaustion was not enough to induce him to pass over his long soak for a much safer shower in those instances. The first time he was caught - startled awake chin deep and slowly sinking in lukewarm water - Jim chastised him ('Really, Seb, if you're going to get yourself killed, at least do it on the job; drowning in the flat is such a waste.') but the behavior didn't change. Sebastian asserted that a long hot soak was what he needed after jobs like that; a hot shower just didn't do the same job for aching muscles and the occasional bone-deep chill.
Jim hadn't said another word about it, and he could tell Sebastian thought the matter dismissed as irrelevant, though he had simply chosen a different tack. Orders were made, and during the next overnight trip out of the city (Wicklow this time; nothing grand, but someone who was meant to be dead in London the week before had apparently not gotten the hint – or perhaps had gotten it too soon) Jim went about testing the effects of his chosen plan.
After a bit of searching, he had found a pigment that would stain water an appropriately horrifying shade of red without staining the occupant. While Sebastian was away, he ran four separate tests to verify the right quantity of pigment to use for the most appropriate water levels, then made sure to thoroughly rinse out the tub again. Sebastian suspected nothing when he returned.
The plan then went into a holding pattern; waiting for the right time to present itself. It was another month before Sebastian had that sort of job again, and another three before he was caught asleep while bathing. This time, Jim did not wake him, merely entered the room on quiet feet and carefully added red liquid to the bath. It dissipated slowly, but effectively, and a careful swish of his hand through the cooling water hastened the process without ever alerting his slumbering companion. Once his task was finished, Jim smirked, satisfied with his work, and removed himself just as quietly as he had come, to read within hearing distance.
The wait was not long – ten minutes and half a chapter in, he had results.
Jim heard a gasp, a sudden, brief sloshing of water, and then utter stillness. The brilliant thing about Sebastian was, he doesn't panic - Jim was very nearly convinced he would take a mortal wound with little more than a disgusted curse and finish what needed be done – the silence from the other room only solidified that belief, and a slow, approving grin curled his lips.
A minute later – long enough for a semi-thorough full body check – a low growl sounded through the door, and a yelled, "Goddamnit, Jim, give me a heart-attack, why don't you?!" rumbled through the flat. Jim simply smirked in response, sauntering past the bathroom door and replied in a darkly teasing voice just above normal speech, "I warned you sleeping in the bath might kill you, Sebastian; never said you'd drown..."
The habit wasn't completely broken, from then, but Sebastian did start taking more showers.