I haven't gotten the "Artwork of Return of the King" book (though I have the Two Towers one) Looking at the costume for Denethor, I can see a couple of elements that remind me of two of my other favorite costumes, for other afflicted/misunderstood characters:
Denethor is wearing chainmaille, like the warrior king Theoden, but over it is a very baggy sort of bathrobe-like robe. For the King Theoden costume, Ngila used a bathrobe-like coat for when Theoden was afflicted and possessed by Saruman. Denethor is also afflicted, not through possession, but by complete despair, which has totally consumed his ability to be a proper leader to his people, or even a proper father to his surviving son. In profile, Denethor's robe is also like Grima Wormtongue's cloak. Ngila said she wanted to give him a "hunchback" appearance to make him look deformed and frightening, to reflect his mental state. This same look works for Denethor, for similar reasons.. it also conjures up the idea of hiding down within himself (like a turtle in his shell) and not being able to confront the changes that are happening in the world around him.
Does anyone have the official book for this film? There must be lots of good stuff about the designs.
John Noble has extremely powerful shoulders & the whole effect of the costume (particularly when he walks through the doors to the burial vault) is very big, strong and unstoppable.
The design for Minas Tirith was based on a classic painting called "The Tower of Babel", I think (??)