"Sherlock" / cab scene: Breakdown, subtext, and analysis
Context: While Sherlock and John were looking over/moving into 221B Baker St., Lestrade showed up and made noises about there being a corpse he wanted Sherlock to look at. Sherlock swung the coat on and dashed out, only to immediately come back in and ask John to go with him, on the pretext that he really hates Lestrade's forensics guy and he wants to make a point by showing up with an assistant. The two of them catch a cab, wherein John finally asks Sherlock exactly what he does and why the police are asking him to come poke one of their cases.
[Subtext to the context: When Lestrade shows up, John gets to see Sherlock really being Sherlock for the first time -- he's more thrilled than is normally considered seemly at the idea of a murder, and Lestrade seems strangely resigned to this behavior. What he doesn't realize yet is how un-Sherlock Sherlock is being already. He's never dragged an assistant to a crime scene before; turning around and asking John to come with him seems to be an impulsive act. John also doesn't yet know the extent to which Sherlock either fails to pay attention or fails to put into proper context a lot of the stuff he reads off of other people. His only experience with his new flatmate so far is Sherlock being in 'charming people to get what I want' mode, when they were first introduced, and now showing a kind of uncanny instinct that the way to get John to come stare at a corpse with him is not to apologize for the bother and ask for his expertise, but to promise him adventure. Sherlock himself either doesn't realize how much of the deduction is the same as people-reading, or he was making a very risky blind guess on that one.]
Sherlock: When I met you for the first time yesterday, I said 'Afghanistan or Iraq?' You looked surprised.
John: Yes, how did you know?
Sherlock: I didn't know. I saw.
The second thing of note is that John steps on the end of Sherlock's line -- he's been wondering about this and is really curious to know WTF was going on there, and he didn't know how to bring it up but look now Sherlock's talking about it and how did he know anyway? The fact that this thing is a valid conversational topic is not news to him. It stuck with him, and he's been turning it over in his head since it happened.
Trust me, people don't want to know how you do this most of the time. Most of the time they actively resist hearing the explanation. It creeps them out and they don't want to know. Almost always, when people ask "how did you know that?" they don't actually want to know how you knew that -- they're angry that you know something they weren't prepared for you to know and they want you to sputter or provide a mass of sloppy guesses they can pick apart, so they can prove you're not so magic after all. In person especially, it's vanishingly rare when someone actually wants to know.
Sherlock launches into his explanation here. Cinematically, a large part of it is flashback to their first meeting in the lab at the medical school (referred to as "St Bart's", it was the medical school attached to St. Bartholomew's Hospital in Doyle's day, today called Barts and The London School of Medicine and Dentistry, one of the oldest medical schools in the UK). The bits that aren't seem to show Sherlock lecturing largely to the empty air in front of him in the cab rather than at the good doctor. John is not overly bothered by this, mostly because he's not looking at Sherlock much either; his eyes are bouncing from place to place in a way that's normally considered indicative of someone thinking furiously. Throughout the series, John continues to not be bothered much by the lack of eye contact. He's generally normal about it with people like Lestrade, but with Sherlock he copes by doing exactly what Sherlock does, which is addressing air in the same general vicinity as his flatmate and figuring he's either listening or he's not, eye contact be damned.
Sherlock does swing over and look at him when he wants John's phone back for a demonstration. John is paying enough attention that he's not taken unawares by this; in fact, he hands over the phone so readily and automatically that they don't even bother getting it in shot. This is not the reaction of most people when you've just told them that you figured out their whole family history from a small everyday item they carry on their person. Usually, once you've pointed this out, they take it off/jam it deep into a pocket/shove it behind them on the seat, out of some vague near-superstitious feeling that you're going to use it to read the deep dark recesses of their mind. John hands it over and pays fucking attention as Sherlock, eyes fastened on the phone, turns it over and points at things.
John is also thinking fast enough that when Sherlock says "Next bit's easy, you know it already," he realizes that Sherlock is talking about getting names off the engraving on the back of the phone. You really have no idea how many times I've said that and then had to remind people that there is some sort of writing on their doodad, and that I am fully literate. It's one of those things that evidently just slips peoples' minds very quickly once they get used to owning an object.
John keeps his eyes on Sherlock's face as Sherlock, still holding the phone, looks at nothing in particular. Sherlock is actually being quite strangely charismatic, for Sherlock; normally when he explains things to people, his audience is either spooked or hostile. For once he's got a student who isn't trying to crawl into the far corner of the cab as he's talking, and he's liking it quite a lot. Normally, Sherlock is impatient and even a little defensive when he's elaborating on the deduction, but this, I think, gives John some insight into just how enjoyable Sherlock finds the act of putting these things together, and I suspect it makes it a lot easier for him to accept later on that Sherlock is not losing track of other people's feelings out of sheer callousness.
(He's also a bit amused. You don't learn it until the scene after this, but Sherlock is actually wrong on something here: "Harry" is short for "Harriet". He has no way of knowing that; even if he had been searching John for reactions the way John is searching him, John doesn't give anything away.)
Sherlock does note briefly that if Clara had left Harry, Harry would have kept the phone out of "sentiment". He's not thoroughly ignorant about how people work, but he does fail to recognize at several points that his ability to deduce things, and his general knowledge of how various motivations make people react, are together the same thing as having a gestalt sense of people-ness. There are a number of reasons this might be -- it's possible that it's an autistic behavior, but there are several points where he reacts more conventionally to things John says or does, which makes me think that a lot of it is due to him coping with extreme social isolation by devaluing what he didn't have and pouring his entire self-worth into what he did.
John: How can you possibly know about the drinking?
Sherlock: Shot in the dark. Good one, though.
When Sherlock finishes, he falls silent. This is the part where most people swear, or gape at him, or just flat refuse to talk to him on the grounds that they don't want to give him any more material for mind-reading. I've had people do all of the above to me. It has nothing to do with rudeness, although that doesn't help Sherlock, but more with people being suddenly confronted with the discrepancy between how much information they think they're leaking and how much they're actually leaking to someone who knows what they're doing. It's a bit like suddenly discovering that the guy you've been sitting across from on the bus for the last hour has x-ray vision and realizing he's been staring at your underpants the whole time and, worse, not caring.
John pauses and then informs him, in a very impressed but also very matter-of-fact tone: "That was amazing."
Note Sherlock's reaction here: It takes him a moment to speak up. He's expecting something more, probably something sarcastic or biting. He ventures to ask for confirmation and John runs over the end of his sentence again, affirming with no hesitation that that was, in fact, extraordinary, and he's not saying it grudgingly because he needs Sherlock to brain at something important.
Sherlock: That's not what people usually say.
John: What do they usually say?
Sherlock: 'Piss off'.
The reaction from both of them is to smile. This is probably the first time in a very long time, if ever, that Sherlock has gotten this from someone else: someone thinks his talent is fantastic, not terrifying, and is genuinely sort of perplexed at the idea that anyone would think otherwise. It's also fairly rare that Sherlock has any confidence in his read on someone else when he's not looking straight at them; in this case, they share the sentiment while both turning away. It's debatable whether Sherlock really can't figure people out without looking, or if he's just so determined to be deductive that he won't let himself draw conclusions from other cues, but he's most successful at both, by far, with John.
The subsequent bits at the crime scene are also interesting, although if you think I'm going to go hunt them down on YouTube you're out of your mind -- it took me half an hour to find that one among the welter of fanvids, and that's one of the more famous bits from the very first episode. Their interaction over the body gives Lestrade pause more than once, particularly when John accidentally blurts another "that's amazing" after Sherlock has already slammed a door on his least favorite crime tech and told several other people to shut up. He asks if John realizes he's doing it out loud, and John apologizes; after a short silence, Sherlock, sounding perhaps a little confused, says, "No, no, that's... quite all right."
Lestrade watches this in relative silence. I'd like to point out that he is one of the few people who does not make assumptions or snide remarks about the two of them shagging. Lestrade has been dealing with Sherlock for long enough that he'd be in the Holmes family portrait, if the Holmes family could stand each other long enough to have one taken and Lestrade didn't have enough sense to bolt in the other direction the moment anyone thought to ask. He seems to recognize that whatever is going on here is unusual for Sherlock and probably going to become important. He doesn't say anything to Sherlock directly, but it does dawn on him at some point that John doesn't know what Sherlock is like without him, and he does say a few things privately to John that indicate he knows something is changing, and he's not going to make any assumptions about exactly what it is. He and Mycroft also realize eventually that Sherlock listens to John like he listens to no one else, although Mycroft tends to use it for evil, or at least for annoying.