It was surprisingly easy to anticipate John's reactions - even provoke them. Most other people weren't worth the bother. They were too boring. But John... He had potential. He interested Sherlock like few had before. And there was something strangely compelling about how he never seemed to notice when Sherlock manipulated him.
Ordinarily, that was boring too. No one could keep up with him; Sherlock had come to expect it. He didn't know why it was endearing on John when it was so annoying in everyone else. Perhaps it was John's honest and open admiration and awe in Sherlock's abilities. He didn't get that from anyone else. The police would call him, listen to him, but they didn't praise him.
John's admiration was the first emotion he set out to elicit. He started explaining more of his thought processes, working his way through each of the clues out loud so that John could follow and be amazed. It never failed, and Sherlock basked in its warmth.
John's protective instincts were the next to catch Sherlock's attention. Of course he had them; it had been obvious from the first time Sherlock met him. Like John's inability to keep up mentally, Sherlock didn't find it nearly as annoying as he would the same trait on someone else. John didn't try and stop him from working, from investigating, from anything. He didn't even say anything about Sherlock wearing multiple nicotine patches at once, though he did have a very specific disapproving look for it. The doctor in him, Sherlock supposed.
No, John's protectiveness showed itself in much more pleasing ways. By always being there, by having his gun with him even when he really shouldn't, and on one occasion, by telling Sergeant Donovan to piss off.
Sherlock smiled at that memory. That was when he really started appreciating John's protectiveness, though John himself must have apologized to Donovan a hundred times.
Sherlock never had to rouse John's protectiveness. It was always there, quietly watching and waiting, like John himself. Sometimes, though, he courted a show of it, analyzing a situation and putting himself in the least dangerous position that would appear dangerous to someone who just couldn't quite keep up - like John. And perhaps Sherlock could admit, if only to himself, that once or twice the situation actually had been more dangerous than he had anticipated.
John got angry those times. That was the one emotion Sherlock never even tried to bring out, but he still saw it more often than he liked. Like everything else about him, John's anger was different. It wasn't like Lestrade's, or Mycroft's. Sherlock didn't care about their anger unless they tried to make him do something. But John...
John didn't normally say anything about it. But he looked, and Sherlock soon grew to hate that look. It was never just anger. There was fear, and disappointment, and something else that Sherlock could never quite catch because John always turned away too quickly when he noticed Sherlock analyzing him.
The first time he realized what it was, John wasn't even angry. They were in the flat, between cases, and Sherlock was playing his violin while John pretended to blog, but spent most of his time watching Sherlock instead. It was just idle observations to start with: John's eyes were dilated, though the room was well-lit; he was flushed, though it wasn't particularly warm; he couldn't seem to stop watching, though nothing particularly fascinating was going on; he was sitting so as to keep his lap from Sherlock's view...
Sherlock almost dropped the bow as he realized what the emotion must be. John... No. No, he was wrong. Could he be wrong? He was hardly ever wrong. He probably wasn't wrong this time. Probably. But he could be. Always a chance. But this was John. He knew John. He wasn't wrong.
"Sherlock?" John had his concerned look on, his forehead wrinkling just so.
Sherlock considered his own thoughts for a moment, not answering. Did he feel the same about John? He did get a certain physical response from being around John. And there was no denying that he reacted differently to John than to others. Was that enough?
He wondered what response a kiss would get, from both of them. It was odd not knowing. Was this how John felt all the time? How depressing. Well, Sherlock wasn't going to put up with not knowing something, at least not when it was important. He set his violin aside and took two steps to cross to where John was sitting, bent down, and kissed him.
John made a muffled, startled sound, but he didn't pull away. After a couple of seconds, his hands came up, wrapping around Sherlock and pulling him down to a better angle.
When they broke apart, Sherlock dropped into the closest chair, taking stock of the effect the kiss had had. Satisfactory, he decided. Most satisfactory.
John looked at him, his 'what are you up to now?' look. "That better not have been one of your mad experiments," he said, slightly breathless.
Sherlock just grinned and leaned in for another go. Perhaps it was. But at least it was an experiment that they were both enjoying.