This so much! Like it's all wrong, Jack is supposed to be the reckless, rogue one and John is supposed to be soft and sensitive, never liking confrontation.
It's funny how reading fics creates a narrative of strangers in your head that can be way off.
I think they are broadly both like that - some of the details of our impressions come from fics of course but the broad strokes are from how they present themselves to the world.
But no-one is as simple as just their public persona, and I feel like that one incident (high-emotion, heat of the moment stuff) taps into parts of both of them that are always there but usually beneath the surface.
So John, who's normally slightly awkward and perhaps a little reserved, who tends to approach most situations with good-natured humour, bypasses his conscious thought processes and acts on pure, unthinking, righteous anger.
And Jack, who can often be reckless and thoughtless and able to freely and without guilt or complications express his negative emotions in the moments they occur, manages to show some sensible, clear-headed restraint and act as an agent of reason and good sense.
What's really compelling is the way they're both simultaneously acting out of character (with how you'd expect them to act, on the basis of how they normally act in public). Also the combination of the gentleness of the grab itself - Jack's making no demands, it's a light touch that looks easily escapable, it seems to be more a grounding gesture than an order or insistence that he turns around - and the ease with which John acquiesces, turning back immediately just from that one simple touch. Makes me think they do have a pretty close connection, a friendship based on a fairly deep understanding.
It's just such a small moment that's endlessly compelling.