You take your time. Get yourself together. I want your best game.” ~ Jeff
“It’s not a game. It’s chance.” ~ Sherlock
“I’ve played four times. I’m alive. It’s not chance, Mr. ‘olmes, it’s chess. It’s a game of chess, with one move, and one survivor. And this ~ this ~ is the move. Did I just give you the good bottle or the bad bottle? You can choose either one.” ~ Jeff
Jeff is clearly right-handed, we see it by what pocket he keeps his phone in, the hand he holds his gun in, and the way he cocks his head. When he first appears in the flat, he has his right hand in his pocket and you can see the outline of the pill bottle in his left pocket. It would make sense for him not to want to touch the poisonous one, so therefore the bottle in his left pocket could be the bad one. However, every time he reaches in his pocket for his phone, and knowing that one of the bottles contained a pill that was harmless, it could be that instinctively he wouldn’t feel the need to keep touching the bottle like he does, and therefore the bottle in his right pocket could be the bad one.
At the college, he takes out the pill in his left pocket first. It could be that he wants to get the poisoned one out of his pocket first, or at least that’s what I would do. If I had two pills in my pockets and one of them was poisoned, I’d want to get that one away from me as quickly as possible. However, it could be that that one was the harmless one, because looking at it, it’s just one pill, and his opponent doesn’t know yet (he’s assuming) that he has two pills. When he takes the pill out of his right pocket, however, he lets go of it slightly more slowly than he did the first one.
When he says, “Whatever bottle you choose,” Jeff glances briefly at the bottle on the left. It could be because that’s the bad one, because he’s hoping that’s the one Sherlock will choose. He slides that one towards Sherlock; this could be that he wants to get the poisoned one as far from him as possible ~ we saw it in Princess Bride. However, like Princess Bride, he knows his opponent is clever, and he knows how people think–he knows his opponent would most likely take the pill in front of Jeff instead of the one offered to him, therefore the pill in front of Jeff could be the poisoned one. Jeff also keeps glancing at the pill in front of himself, and this could be a natural reaction, always wanting to keep an eye on the danger. As well as that, a person naturally, when possessing two items, would want to keep the better one for himself, and so the pill in front of Jeff could be the good one. But in a matter of life and death, a person wouldn’t take what is offered to them, because they know the other person would want to live as well; and though he is dying, Jeff sends money to his kids for every life he takes, so he still wants to live as long as possible in order to help them, so the bottle in front of Jeff could be the good one. But Jeff is clever and he knows his opponents are clever and wouldn’t take what he offers them, seeing as he’s trying to kill them, so the bottle in front of him could be the poisoned one.
When Sherlock takes the pill in front of Jeff, Jeff doesn’t look upset. His face when he says “Oh, interesting,” shows nothing. Saying the word “interesting” could mean that Sherlock chose differently than the other four, and therefore the pill in front of Jeff could be the good one; however, it could also mean that Jeff is disappointed that Sherlock chose the same as the others, and therefore it could be the bad one. Jeff’s hands don’t shake at all when he holds the pill that was in front of Sherlock, so that pill could be the good one; even a dying man would fear death to a degree, and he’s watched the other four writhe in pain as they take their poisoned pills. Even a dying man fears pain, but there is no sign of fear in Jeff; he is confident in his genius, as it has killed four people already. And when John shoots him, Jeff is obviously shocked. He wasn’t planning on dying tonight. But when asked if Sherlock was right, Jeff is unwilling to answer. It could be that shock makes him unable to reply, but it could also be that he doesn’t want to admit that Sherlock won.
If he wanted it to be more interesting for his victims to choose a bottle, Jeff should have not moved one forward at all, because they would have nothing to go on. However, that would not be as much of a game, and would in fact be just chance, because “you’re not reading the numbers, you’re reading ME.”
Now, going slightly off-topic and to be perfectly honest to John, he does seem in fact to be “very loyal very quickly,” as Mycroft said. This entire episode, after all, is over the course of only two days, with all the action happening in one day, and John does seem to…how shall I say…”latch on” quite quickly to Sherlock. The way they interact is not how normal people interact when they just meet somebody.