When John moved in with Sherlock, he had to make quite a few adjustments to his previously mundane style of living.
For one, he'd had to learn to cope with seeing bits of human organism in their fridge (not particularly a problem, when his background was taken into account), and these said organisms being placed precariously close to consumables- for the "purposes of science!", or so Sherlock claimed. And whilst this didn't sit well with the trained medic within him, John had learned to adapt and deal with this. ("Sherlock, if it's had acid on it or disease injected into it, it's going in the freezer that doesn't contain the things we eat.")
His subtle changes and tweaks, equally reciprocated by little changes of Sherlock's own design, were to an outside perspective small and simple things to ask for, but to Sherlock were extravagant, sprawling cracks in the carefully crafted structure that made up his living space, had improved the general quality and efficiency of both their lives- Sherlock now took a shower daily, and consumed at least a meal and a half a day, (Before, Sherlock had eaten a couple of biscuits and cups of tea- just enough to keep his 'transport' functioning.) and John went grocery shopping regularly, putting up with Sherlock's demands for various brands of exotic foods, pieces of machinery, and, one memorable time, several different types of sequins, with a well practised sigh that spoke of years of the same routine.
In reality, John had only been with Sherlock for 8 months at the time, and before he'd moved in, John had barely gone grocery shopping once a month. Moving in with Sherlock had ignited a determination (one that was passionately nourished by Mrs Hudson) in John to make sure that the both of them were healthy and fighting fit; they had to be, with the level of inter-city chasing and wall-climbing they did!
But keeping Sherlock fighting fit was always a problem when he was engaged in The Work. Sherlock focused on the case with such a terrifying drive, emotionally detached and yet physically so engaged, that he refused to take a break for anything, for anyone who wasn't John, Mrs Hudson, or Lestrade. The only time he would take a momentary pause was to explain something to the dimwitted humans around him, lest they continue to pester him with 'why's and 'how's. And John had a problem with this, because whilst the lack of showering and sleep weren't of too great a concern for him (Sherlock's body would eventually force him to sleep, and showering wasn't necessary for survival. Besides, he could always throw a bucket of soapy water on Sherlock whilst he brooded on the sofa), the genius detective's refusal to eat was.
It was completely and totally unacceptable for Sherlock to not eat for periods of longer than 9 hours at a time, and no matter how many times John told him, Sherlock honestly could not help it. His dedication to his job took such precedence over everything else that he simply could not make his mind find space for things like food, pre-programmed as it was to finding solutions and testing hypothesises. Sherlock had stated many a time that concentrating on chewing and swallowing and tasting and digesting took valuable processing power away from The Work, and that was not allowed in any way, shape, or form. And so, it fell to John to ensure that the high functioning idiot actually ate.
In the beginning, John had tried pestering Sherlock. He'd set himself up in the living room, where Sherlock would be curled up on the couch, eyes staring unseeingly at the bullet hole riddled wall in front of him as his mind raced at a thousand possibilities per second, and asked Sherlock to eat something every ten seconds, waving plates of toast underneath his nose and doing all he could to distract him. Sherlock had responded with a glare powerful enough to make a lion quiver, and not even quailed at John's own response; a glare he'd picked up from his commanding officer in the army back when he'd still been a private. John had debated on forcing Sherlock's jaws open and shoving the plate down his throat, but that would just earn him copious amounts of sulking, and Sherlock was a pain in the arse when he sulked, even if it was in the (slightly) loveable way.
But John had not given up on his mission. He'd tried mixing it in with other, seemingly innocent case related questions. He'd tried putting plates of food in places that Sherlock would most definitely touch, expecting to find the leather that he'd just been examining, but instead getting rice over his hand. (Sherlock's ability to notice things that didn't pertain to the case decreased worryingly dramatically at these sorts of times) Hell, he had even sat Sherlock down and told him that he was not letting him retrieve the results from an important experiment until he'd eaten something! (Sherlock had called Molly. Molly had lasted all of 40 seconds before she spilled the beans.) And yet, it took him another month before he finally figured out just how to make the curly-haired man eat during a case.
John had always known that Sherlock was a bit of a drama queen, who would do almost anything to prove that he was smart, that he was amazing, special, a cut above the rest. He had seen it in the way that Sherlock had always, always answered John's questions at a crime scene, after the customary sigh that meant 'Really, you can't see it? But it's so simple!', had seen it in the way that Sherlock's eyes always lit up, just a little bit, when he was praised (most noticeably when the praise came from John himself, the ex-Captain had long since realised). If there was one sure-fire way to get Sherlock to open his mouth, it was to get him to explain something. And really, at that point, the idea of just shoving food into his oral cavity was looking more and more appealing by the case, and once the food was in Sherlock's mouth, the man would swallow- or risk getting food over his precious scarf and coat. (Oh, and there was an idea forming in his head, one that sounded just a little bit ridiculous. But then again, one had to be a little bit unconventional when dealing with Sherlock Holmes.)
So John had ordered some fried rice, boxed it up, and taken his usual place at Sherlock's side, waiting for his moment to strike with a tablespoon, a weapon of mass food-deliverance. For Sherlock, it had happened too quickly. One moment he was describing the difference between whip marks made by an actual whip, and whip marks that came from a piece of leather- and just how that related to their suspect-to a note-taking Lestrade, (The lines are finer, created with more finesse. Besides, the evidence suggests he was experienced with this sort of thing, and do you really think that a millionaire would settle for something that was free?) and then suddenly there was the unmistakeable taste of soy sauce, rice, and egg in his mouth.
Chewing thoughtfully for a couple of seconds, (From the Chinese place down the road. John's doing, then.) Sherlock sent a vaguely injured look towards John, which was answered by a shrug that said 'it was going to happen sometime', before swallowing and returning to his lengthy explanation. Mentally, John performed a very dignified jig of victory, proclaiming his mission a success with many benefits to follow.
And over the cases, John continued to feed Sherlock in this way, irrespective of who he was talking to (unless it was actually someone really important, because Sherlock had a reputation and it mattered to John). Donovan's scathing comments of 'What, can't the Freak even feed himself now? Or have you turned into his servant, Watson?' were met with John's cold reply of 'This man doesn't eat, doesn't sleep, all to do your dirty work, from which you get promotions and praise. He doesn't ask for recognition, he doesn't ask for money. He gives himself to this city, and it's the least I can do to make sure he actually eats, seeing as unlike you lot, he doesn't take lunch breaks!' It's slightly fanciful and slightly untrue- Sherlock doesn't give himself to the city, he gives himself to The Work, and Donovan and John both know it, but it sounds good and makes Donovan shut up, so that's all that really matters.
And the very subtle, grateful glance that Sherlock sends him definitely doesn't make him feel warm and happy inside, as he listens to the detective launching into an explanation about the various stains different sorts of tobacco ash leaves and readies the spoon that's become a constant companion to his hand. The more details he included, the better really- John had more opportunities to slip food into Sherlock's mouth. Hopefully he'd go through all... what, 247 types?
John loads the spoon up with pasta- his ammunition – and prepares to fire.