This is toward the end of the episode, after the "drugs bust" and Sherlock running after the cabbie and leaving John behind (again), and also after (again again) declaring Sherlock a hazard and the Scotland Yarders giving up on tracking down the phone. Mrs. Hudson is gone, too; it's just John and the computer trying to trace it. There's so many parallels between the first moment in that flat when Lestrade tries to get Sherlock involved in the case and John sems a hair's breadth away from deciding to stay at home and watch telly. You know, accept the banality of the only life he has available to him. He's even grabbed his cane to get around the flat.
And then he gives the laptop a second glance, and he runs out the door to find Sherlock. All that's the basic plot summary. For some reason, what stood out to me was the camera-work, what the scene focuses on. For a good while there, John's run out of the flat and we're just stuck there looking at the room he leaves behind until Sherlock and Hope open the door to the college hall and wipe the scene away. One thing I've learned about this show is the cinematography isn't an accident, what they linger on unnecessarily, the way they frame scenes, often holds the key as much as the words and actors' facial expressions to what's important. And here we're left looking at... a chair. Specifically, John's chair with its union jack pillow. This is what John's leaving behind when he chooses to run after Sherlock.
So that got me thinking about just what this chair represented. If it was just the chair I'd say this is about John leaving quiet domesticity (clinic hours and crap telly over take-out, for instance) behind him, but there's the Union Jack pillow as well. That could mean a quiet life, I guess, but to me at least it speaks more to national pride. Queen and country. And John could have that kind of thing if he's just a pillar of the establishment, a less well-off version of Mycroft's public face settling down into that quiet, decent life - but he also had it with the army. Being a "good" British man doesn't necessarily mean quiet and dull nights at home; John's army adventuring would also probably fall into that category. So when John leaves the pillow behind, when we're left lingering on his comfortable chair decked out with such a symbol of respectability... I have to think it's that, more than a life free of adventure, he's leaving behind when he runs after Sherlock. It's looking at loyalty and what's right more than what's legal or permitted. If that makes any sense.
Maybe I'm making much ado about nothing. I'm still very new at reading literature and cinematography, and I always seem to read the symbols the wrong way, or at least the not-quite-ordinary way. I kind of wish I could read someone better at these things take a whack at this scene. Still, those are my thoughts on it and I thought some of you might find the moment interesting to think about.