John began to observe his flatmate a little more closely during their interactions and so discovered a few things which he never really thought about before.
The first of which being that Sherlock drank a surprising amount of non-blood fluids. And not just surprising for a vampire. John was sure people living in the desert with raging colds didn't drink half so much fluid as he did. There were times when he'd walk into the kitchen to find Sherlock chugging an entire gallon of milk.
The second thing John noticed was that Sherlock didn't seem to have any trace of a gag reflex. He tried not to dwell on this too much.
The third was that his vampire flatmate spent a suspicious amount of time with the police detective named Lestrade. Every few weeks or so, he'd come by and visit. Sometimes he'd have a file for Sherlock to puzzle over, but usually he didn't. Every time, they'd both disappear into Sherlock's bedroom for hours. After which Lestrade would stumble out with rumpled clothing and strangely bright eyes and Sherlock would spend the rest of the day in a hazy cloud of self-satisfaction.
John was sure he knew what went on between them. But his mind refused to let it go.
The detective was sitting at the kitchen table after one of these strange rendezvous, looking gray and foggy like dawn breaking in a forest carpeted with autumn leaves. He had a glass of orange juice firmly clasped between both of his hands, as if he were afraid it'd escape. "Afternoon." He mumbled amiably.
"It's nine o'clock." Corrected John as he pulled a carton of leftover chinese out of the fridge.
"Oh." Lestrade glanced at the darkened windows with surprise. "So it is. Funny how that happens."
"Time?" John chuckled, picking at his possibly expired orange chicken.
"That too." Lestrade smirked over the rim of his glass of orange juice. He lowered the glass, squinting philosophically at the pulpy fluid "It's like… coming out of a cinema to find that half your day is over. And it didn't feel at all like that much time had passed."
"Hm…" John nodded thoughtfully around a bite of definitively expired orange chicken. He discreetly spit it back into it's carton. "So, you and Sherlock."
"The hundred year old toddler, yeah."
"Are you two…" John fumbled for the least awkward euphemism. "Together?"
Lestrade stared at the space between them, as if pondering the history and evolution of the term 'together'. "No. I'm married." John noticed that Lestrade's eyes weren't glazed over or unfocused like one might have expected, but bright and alert. Almost too alert. "Why would you assume that?"
"You two disappear into his room for hours…. You come out looking like- uh. That. It's kind of difficult not to." John muttered awkwardly, pushing the carton of expired orange chicken to the side.
"Oh… yeah." Lestrade cringed. "I just feed him. I never actually realized what it looked like."
John chuckled to relieve the tension in the room. "So this is all the blood loss?"
"Nah, he only took maybe half a cup. This is all the vampire saliva." He said, carefully lowering his glass to the table like it might jump up and bite him.
"Hm. I didn't realize it took that long."
"It doesn't. No, the actual bloodletting only takes a few minutes." Lestrade picked up the sports section of the newspaper with all the methodical caution of a brain surgeon and eased his chair back to read it. "The rest is just watching Sherlock sleep and hoping he'll roll off my arm before it falls off."
John wasn't sure exactly what to make of that. So he got up and rummaged about the fridge, despite the fact that he knew there was nothing in it.
Lestrade laughed humorlessly behind him. "I just realized something." he said, with the air of a man waking up in a jail cell, remembering he's on death row. "I don't think I'm going to be married for much longer."
"Oh? Why would you say that?" John asked sympathetically.
"Partly because of my secret life as a blood servant to a family of ancient supernatural beings. Partly because of my dedication to my demanding and very dangerous career. And I'm not going to pretend our declining sex life has nothing to do with it. But.. mostly because I keep running off to the flat of a gorgeous young man and coming back hours later looking dazed and happy." He laughed at his own tragic joke, gingerly setting the newspaper aside.
"Sorry to hear that." John mumbled, sitting back down to console his roommate's main course.
Lestrade shook his head like he was afraid his head might fall off. "I don't blame her. I might as well have been cheating on her for all these years, what with all of the secrets I've been keeping from her. And Sherlock, god. We're not... like that. But I've put more effort into caring for that needy bastard than my own daughter." There was a pause
"Sorry. You didn't need to hear that... -it's the vampire saliva. Effects everyone differently, but it makes me a soppy idiot."
"No, it's... fine. Do you know how many times I've drunkenly sobbed about my first girlfriend to whoever happened to be at the bar?" Lestrade shook his head with a morose chuckle. "Yeah, well neither do I."
Lestrade took another thoughtful sip of orange juice.
"What's it… like? I mean, if it's not too much information. I assume it's not quite like getting your blood drawn."
Lestrade sighed, the way people do when they've answered a question too many times and still don't know the correct answer. "Different people have different experiences. Some say it's almost religious, others say it's like drugs. There's a whole cult of freaks who see it as erotic. And of course, every vampire is different too. But mainly it's like being drunk."