"Reading people" doesn't require interaction. They, as you say, apply cold-reading style techniques (and other techniques, of course) to the selection of evidence and testimony available in the case.

In the end, though, they are similar to the "psychics" and confidence men; they need to quickly discover some key insights into a person's personality from (often VERY) limited data.

Also note: the very best cold-reading grifters/con-men don't have to interact with the suspect ("mark") either, sy least initially. It's part of how the mark is selected in the first place.