I think the point is that expressing interest is not sufficient enough. There are plenty of examples where people 'express interest' in something but don't go out of the way to act differently with it.
I believe what Scott identifies as "unexpected and physical" is actually better understood as "sufficiently hyped". For media businesses like comics, games, etc., hype is a thing you can explicitly build into the design of the product, with elements that are audience-identifiable and build anticipation: will this protagonist-that-resembles-me survive? what is the secret to the mystery? which cutie will they get with at the end of the story? Will the strategy I'm going to try for beating this game's boss work?
With businesses that aren't squarely entertainment products, the same kind of hype is built with careful marketing and branding. Red Bull maintains its brand not by saying it's scientifically better than other energy drinks, but by showering the landscape in an impressive array of marketing stunts that are intrinsically hype-y(who wouldn't want to learn about Felix Baumgartner's space jump?).
Once you've built the hype, people cross a certain kind of "activation threshold" and start going out of their way to talk to you. They're fans. That's when you see "unexpected and physical".
"If people express interested or thank for it unasked" might fit better.