Based slightly off of my own experiences with severe ADD, something I personally think Sherlock might have, although his intelligence would make it worse than I've ever had to deal with. Please let me know how I did!
Sherlock Holmes is staring at the wall.
Fingers tented against his lips, knees drawn to his chest, he has not moved for at least three hours.
John is worried, probably. Sherlock vaguely assumes.
Because today is a bad day. Today is the reason Sherlock turned to drugs in his younger days. Today is a day when Sherlock finds himself unable to harness the massive generator that is his mind, and it is charging around his skull like every herd animal Sherlock has never given a shit to learn about.
There is a case: three sisters, murdered in their beds.
Parents ruled out as suspects, nanny ruled out, as were the neighbors and several family friends; of course the assumption was that the girls knew their murderer; this is statistically likely.
The police had ruled out the nanny, but Sherlock wasn't so sure and he revisited the thought again, ever wary of his bias against nannies that formed because Mycroft had always gotten on with the nanny better than he himself had and most of his childhood had been spent in his room reading books like that one child from the tv show that John liked to watch- "crap telly" as the man liked to call it, but why would anyone ever want to spend time watching things they themselves recognized as "crap" unless they just called it crap to appease everyone else who actually didn't like it – perhaps most people actually did like it and just called it crap because they assume no one else does – yes, that must be it because Sherlock had always been a little surprised that John would spend so much time watching it – anyways, in his childhood he spent far more time locked in his room than his parents wanted, but Sherlock had always wondered if parental rights ought to be revoked for parents who were less intelligent than both of their children (yes, just because his distain for Mycroft is immense doesn't mean he was so ridiculous as to ignore the man's obvious intellectual prowess) but on the other hand scientific studies had yet to prove whether nature or nurture caused intelligence so there was a chance his mental capacity was due to those very parents he had begun to ignore resolutely at the age of ten which is just coincidentally the age of the youngest sister who was murdered and OH GOD what kind of failure is he if the stupid irrelevance of his own childhood and crap telly was enough to pull him away from a CASE where lives may be on the line and he just KNOWS John must think he's a terrible person for not making any progress and really John should be able to tell he's not really thinking about the case now, but its not because he doesn't want to solve it and he hopes John doesn't write him off as an uncaring but FUCK DOES HIS HEAD HURT – right across his eyes as if he's been staring at a bright light and it must have been a while since he's last moved because his mouth is dry and his eyes hurt like he hasn't blinked in a while, like he's stuck his face in smoke but the right kind of smoke might actually help if he could convince John to buy him some cigarettes because surely he would understand that healthy lungs were only useful if he was SANE to use them and fuck at this point he would even be glad for a joint– at least it might slow him down for a second because even Sherlock, in all his pride could see that he was useless at full speed if he was going to be like this and oh God those girls are still dead and I haven't made an iota of progress towards finding their killer but why have Greek letters wormed their way into the English language like this and I thought I'd deleted Greek letters because they're FUCKING USELESS AND THESE THOUGHTS ARE RIPPING MY FUCKING SKULL APART AND I JUST WANT TO DIE BECAUSE AT LEAST THEN IT WILL BE QUIET AND MY OWN THOUGHTS WON'T BE SCREAMING AT ME OH FUCK SURELY THIS IS WHAT IT IS LIKE TO BE SCHIZOPHRENIC BUT I AM NOT OH G–
"Sherlock." A worried and slightly panicked voice knocks his rabid-animal thoughts from their path and a cool hand strokes his forehead and pushes his curls back. The other hand works his fingers loose from the vice-like grip they'd formed across his mouth at some point. He becomes slightly aware that the inside of his bottom lip is bloody and swollen where he's been manically chewing and sucking on it for however long and that's surely going to develop into some sort of sore or cyst that the dentist will gripe about next time he's in, which might actually be soon; Sherlock never kept track since John would because that's the sort of thing John does; he's just a good pers –
"Sherlock." At some point the hands had left his skin long enough to obtain the violin and bow from the corner, and had returned to shove it into Sherlock's vacant fingers.
Sherlock takes the instrument and immediately begins to play- something Bach, he isn't paying enough attention to know, but it is familiar, something he must have learnt to play early on in his studies. The music isn't a serum for his thoughts that he can passively allow to work, but more like an escape ladder – one he has to work heroically to stay on, to crawl slowly up and away from the swirling, disastrous mess of his own mind. It's working, though. His breath slows as the music fills the gaps in his mind and keeps the thoughts from running around so much. He can tell because he understands that John is there, had handed him the violin, whereas moments before he had only known it.
Terrified of what might happen if he stops playing, Sherlock transitions into improvisation. It's something mathematical and deliberate that plays and sounds like a hymn, and Sherlock slowly becomes aware of himself. His back aches impressively, as does his neck. His stomach is worked up, and if he'd eaten anything in the last four hours, he probably would have thrown it up by now. He also needs to pee, and guesses that he's actually had the urge for a solid few hours and has just failed to notice.
This is why I delete anything unnecessary, he thinks as he regains some freedom of thought. It's best to let my thoughts hit empty walls than each other at times like this.
He knows his face must be red from the abuse it's been given recently, and his hair is surely a mess. He looks at John, and an understanding passes between them that if they were normal (although they both know their relationship hardly fits into any sort of socio-normative mold even without their personalities), John would have given him a protective hug and Sherlock would be crying right now. But normal is just another word for average, and they are Sherlock and John and not even vaguely average, so this is enough.