He then added that such surveillance would be strange to us civilians. I countered that on the contrary, we knew the principle inside and out. We leap into the story and go about noticing that has to be foreshadowing, and this is a dangling participle. . . .
But when you're doing it for a POV character, that's where it gets truly interesting. Because whatever the character knows is going to color what he sees. A bargeman looks at the river and sees by the swirls that there's a snag under the water just there. A re-enactor looks at an illustration for a medieval theme and says the artist was imitating the 14th century. An equestrian reads a horse's adventures in a fantasy novel and mutters they must really be a kind of vegetable.
Did it right by accident, once. I was reading through a work and realized that when I threw in local color about the plants (down to one scene where they needed dead-heading), it was in the point of view of the character who gardened.
But it's a real trick to keep in the character's mindset because you have to remember it all the time and not reach by reflex for what you would know.