(Yes, I backed up parts and watching them over, being obsessed.)
Sherlock looks very childlike throughout. That scene after the girl screams and only the audience sees his face -- he has that Look of a child afraid of nightmare, that taut painful expression. He's listening to everyone, of course, and Sherlock's face just gets more pinched, more drawn.
And then Moriarty's little message appears opposite. That's when Sherlock looks even worse, despairing; he knows this IS nightmare, one carefully constructed just for him. It's real, the nightmare is happening, he's feeling the trap close around him. Donovan compliments Sherlock, and his face tightens. If Donovan could see his face, she'd wonder -- but she doesn't. We do.
One part I watch over and over is the rooftop scene. When Sherlock is selling Moriarty on not being ordinary, JM closes his eyes for a moment. JM seems truly happy.
Right then Sherlock closes his own eyes too, just briefly, and you can see him thinking, "I've sold him!" The relief. [Made his fast-talk roll perfectly.] When JM opens his eyes, he doesn't see Sherlock do that, because Sherlock's face is composed again. Then JM suicides.
I wonder what Sherlock thought would happen.
I wonder that even more than whether Sherlock really checked to make sure JM was dead. Sherlock must have thought he'd got the winning hand after all... until then.
The first time I watched this scene, I was desperately trying to figure out how Sherlock persuaded JM, because I couldn't see how it worked. Sherlock was willing Moriarty to believe, I realized on later watching. Obviously that "special something" came in handy. :/
Sherlock: We met twice, five minutes in total. I pulled a gun, he tried to blow me up.
I felt we had a special... something.
Which is when Sherlock widens his eyes slightly at Moriarty, who makes his own face back at Sherlock.
Speaking of which, I don't ship. I really don't. But I was looking for that quote above, and found it via TV Tropes Sherlock shipping page. So enjoy. Cracked me up. Still not sold, but I can see how it'd be persuasive. :)
Something Sherlock should've picked up on: JM wasn't afraid when Sherlock grabbed him as if to throw him off the building. Someone not afraid of sudden death in that sort of moment... is terribly frightening.