A lot of writing is plain common sense. I can't tell you how many manuscripts we see where scenes open with conversation between two people, but we don't know where they are. Worse, many times a third person will say something, then following the speech, will be the words, "Danny Martin joined them on the post office steps." It's plain disorienting for Danny to speak, before he joins them. Sort of like someone sneaking up behind you and poking you in the back when you're not looking. And it's even worse if the first two people have been talking for half a page before we find out they're at the post office. Especially if we've already built them a street corner, or a grocery store parking lot in our imagination.
Reading is a participatory sport. It's a partnership between you, the writer, and the reader. In a movie, it's all there for them to see. When a new scene begins, you have an "establishing shot of the post office and the two people coming out the door. Then the camera moves in and they begin to talk to one another. When Danny comes over, we can see him in his postal employee uniform with his mailbag over his shoulder, so we're not too surprised when he joins in their conversation. But in a book the reader takes what you show them with your words, and builds the set inside his or her mind. It's then the writer's responsibility to let the reader know Danny Martin is there, the reader's responsibility to "create" Danny in his or her imagination. That's why it's so important to describe a character when he or she first appears. From the name, the reader will usually infer a bluff, perhaps red-headed Irish postman and would be startled later to learn that Danny was a swarthy complected Episcopal priest.