Sherlock is very much not a tense or anxious person. He is often high strung, sometimes spastic, and almost never just a bit wild, but tense or anxious he is not. Sherlock, over the years of his youth of being tense and anxious, has learned how to calm his nerves right at that first sign of getting nervous. He breaths deeper, trying to smooth his breathing. Sometimes he rubs his thumb against his index finger, but that might just be a habit now. Sometimes he accidentally lets out a lisp, which is a clear sign that he needs to calm down.
John, however, very much is tense and anxious. John comes complete with a full list of nervous ticks and tendencies.
First, that tongue of his never stays in his mouth.
Second, his lips pucker in that way, which as we know has changed to a deep kiss to Sherlock.
Third, he rocks from side to side without noticing. One might see him as simply shifting his weight off his bad leg, but Sherlock -who knows the leg this isn't real- knows that it's time to get John away from said anxiety.
Fourth, he is steady. When John's nervous he can stand, lay, sit, lean, walk, do anything perfectly still.
Fifth, his heart beats in a funny way. What a lot of people don't know, is the night he killed the cabbie, his heart was flying off the handle.
Sixth, he swear. Well, John swears anyway, but when he's anxious or tense, he swears a hell of a lot.
Seventh, he actually thinks clearer. A lot of people work well under pressure, and John sure is one of those people.
Eighth, those days that John just wakes up tense is the days he forgets to put socks on. And he is very much a socks kind of guy, whereas Sherlock is not. The missing socks irks him to no end, mostly because of what little control he has over himself.
Of course Sherlock notices when John's anxious. Of course he notices John's tongue, lips, weight shifting, and odd steadiness. Of course he notices the doctor's heart beating differently, John's uncharacteristically more swearing, John's even more uncharacteristically knowledge of everything under the sun. And of course he notices when John's socks are sitting on the edge of the bed because he failed to put them on before his shoes this morning.
Sometimes, when he's not with John and he knows John is tense, he sends John a text. And knowing John probably won't bother to reply, it might go something like this:
"John, my love, my life, please calm down. Nobody needs to be shot, or fought, or strangled today. You're anxious and I know that, but you need to take a deep breath. I realize you're probably more upset because you forgot your socks, but when you get home they'll be nice and warm for you, along with one of your jumpers. And hurry home, because I'll be sure to be waiting to calm you down. -SH"
When Sherlock is with John and John is tense, Sherlock takes the smaller man in his arms and hugs tight, letting John steady his breathing to match Sherlock's and allow his rapid heart rate to slow. Then, Sherlock simply asks, "What's wrong?" Because John knows Sherlock cares, he knows Sherlock wants to help and is being an uncharacteristic non-sociopath that wants nothing more than to make John happy.