Books featuring interesting characters that come in at over (or around), 400 pages? Very well then. The length requirement will definitely knock a lot of great books out of the running, but the ones that will make the cut will be exactly what you're looking for.
"Journey to the End of the Night" and "Death on Credit" by Louis-Ferdinand Céline. "Journey to the End of the Night" is the greatest book that's ever been written in the history of mankind. "Death on Credit" isn't bad either.
"Petersburg" by Andrei Bely. One of the greatest works of 19th Century Literature that no one has ever heard of, much less read.
"The Long Goodbye" by Raymond Chandler. It comes in at fewer than 400 pages, but it's his longest novel and you won't miss the other 80 or so pages when you're reading it - it's wonderful.
"Victory" by Joseph Conrad. His best work.
"Great Expectations" by Charles Dickens. Dickens was a master at character creation and this book features some of his best ones.
"The Count of Monte Cristo" by Alexandre Dumas. A classic.
"The Tin Drum" by Günter Grass. This one would probably land a place on my list of the 10 greatest novels of the 19th Century as well. Fantastic book.
"Martin Eden" by Jack London. He set out to write a story about a struggling writer that was and was not him at the same time, but what he really succeeded in doing was to produce a book that captures what it really means to be a writer better than almost any other book has ever managed to do. A must.
"Under the Volcano" by Malcolm Lowry. This one is just shy of 400 pages, but despite the fact that the story takes place over the course of a single day it feels much longer. I was a bit reluctant to include it because it's so heavily auto-biographical, but in the end, we're all characters. This is one of the greatest novels ever written.
"The Crossing" and "Suttree" by Cormac McCarthy. Two of the most wonderful books in the catalogue of the man who is quite arguably the world's greatest living novelist.
"Moby Dick" by Herman Melville. The quintessential Great American Novel. There's a delightful poetry to the prose and the characters are outstanding. I love this one.
"Hard-Boiled Wonderland and the End of the World" by Haruki Murakami. My copy comes in at exactly 400 pages. This is one of my favourite books of all time, I've read it more times than I can count. You won't look at the world or your place in it the same way after you've read this one.
"Gormenghast" by Mervyn Peake. It's a given that you'll receive responses from people who will list "The Lord of the Rings" or "A Song of Ice and Fire", but this book has more atmosphere and wit than either of those. Treat yourself.