Since we're getting the story from a first-person perspective (see "Narrator Point of View" for more about that), it's natural that we get to know a lot of the characters through Arthur's descriptions of them. He is often very straightforward and tells the reader exactly how he sees and interprets a person he comes into contact with. Like Jerome:
He might have been anywhere between thirty-five and fifty-seven years of age, with a blandness and formality of manner and a somewhat shuttered expression that revealed nothing whatsoever of his moods or his thoughts. (4.28)
We don't need to read into the way that Jerome is acting, or how he is dressed. We know all we need to know of his personality and demeanor from Arthur's descriptions—as long as we trust Arthur, that is.
Physical appearance tells us quite a bit about one character in the book—the woman in black:
even the swift glance I took of the woman showed me enough to recognize that she was suffering from some terrible wasting disease, for not only was she extremely pale… only the thinnest layer of flesh was tautly stretched and strained across her bones, so that it gleamed with a curious, blue-white sheen, and her eyes seemed sunken back into her head. (4.40)
If this doesn't give you goosebumps—or at least a funny feeling at the back of your neck—you've probably been watching too many horror movies. She looks like one spooky lady, and this is one case when it's okay to judge the book by its cover.
In The Woman in Black, what you are matters. For example, all the humans are pretty upstanding citizens, from Arthur to Sam Daily to Stella. Even Jerome isn't a bad guy, just kind of a coward. And dogs come off well, too: Spider behaves outstandingly in the book and is loyal to her owner and to Arthur Kipps. But it's the other beings that you have to watch out for. The supernatural woman in black has all the malevolence and evil that none of the humans can muster up.