Epistolary, Suspenseful
If you're wondering what the heck "epistolary" means, have no fear. An epistle is another word for a letter, and calling The Woman in White epistolary is a way of saying that it's written as a series of documents. Writing a novel as a series of letters wasn't a new idea when Collins gave it a whirl. A lot of early novels—from the late-18th century onwards—were written in this style, but The Woman in White still stands out because of its epistolary flair.
Some critics thought the novel's epistolary style was a bit superficial. The critics over at Dublin University Magazine snottily noted that "Had the story been wrought out in the old-fashioned way it could have been told far more effectively and in less space." Ouch.
Other critics accused Collins of being overly showy, but the various letters featured in the book actually have some stylistic depth to them: none of the letters are written in the exact same style.
In fact, every first-person narrative we get is written in a style that reflects the character doing the talking. Each character speaks or writes in a unique voice. Check out the contrast between the way Hester Pinhorn and Mrs. Michelson write.
This is Hester:
For one question my mistress asked the doctor about the lady's chances of getting round, he asked a good fifty at least. I declare he quite tormented us all—and, when he was quiet at last, out he went into the bit of back garden, picking trumpery little nosegays, and asking me to take them upstairs and make the sick-room look pretty with them. As if that did any good! (2.4.1.8)
And here's Mrs. Michelson:
The effect of the good news on poor Lady Glyde, was, I grieve to say, quite overpowering. She was too weak to bear the violent reactions; and in another day or two, she sank into a state of debility and depression, which obliged her to keep her room. (2.3.1.99)
Hester uses a very colloquial style, with lots of slang and short phrases and sentences. Mrs. Michelson uses bigger words and longer sentences. She has a more detached tone than Hester, who peppers her account with humor.
Sometimes characters get so into providing details that they start to relive the events they are describing. But for all the details we get, we get a ton of suspense as well. Why doesn't every character just spill his guts to us? Well, Walter kind of instituted a spoiler policy in his story.
When his experience fails, he will retire from the position of narrator; and his task will be continued, from the point at which he left it off, by other persons who can speak to the circumstances under notice from their own knowledge, just as clearly and as positively as he has spoken before them. (1.1.1.3)
Since Walter put a moratorium on commentary (almost like a judge telling people not to get off-track or speculate in courtroom testimony), we end up with a lot of suspense. We only find things out as events unfold. And we get a huge amount of suspense from something like Marian's diary, which was written in the "real time" of the story.