Innevitably though, such detailed character drawing must place a constraint on the author that if violated, as you pointed out previously, frustrates the reader.
Constraints are not always a bad thing. Sometimes they force an author (or an artist) to dig deeper and put in more effort and imagination, instead of continuing on with what is the easiest.
And the author has (at least) two choices that don't require shallow or sketchy characters to keep plot and characterization from clashing.
One is to take the time to create characters who are likely to do the job of moving the plot in the desired direction. If you want them to make stupid choices, that you provide them with the necessary character flaws from the beginning, instead of producing what you need like a rabbit out of a hat when and where it becomes convenient (and then hiding the rabbit behind a prop when you need the character to be perfect again). If you want them to be brave and noble, then you must plant the seeds of greatness early on. Naturally, the circumstances of the plot can promote either the disintegration or the evolution of a character so that the necessary traits, hinted at before, gradually take over.
The other is to create the characters, invent a set of circumstances likely to generate interesting challenges and complications, and then let the imagination run free without the
artificial limitations imposed by adhering too rigidly to a preconceived plot. There are, after all, many different sorts of constraints, and the author chooses which ones he or she plans to work within. The only question, I believe, is which kinds of constraints tend to make for the most interesting and satisfying stories. Obviously, we have different answers to that question.
In addition, such in depth character building, just like world building, innevitably disrupts the flow of the plot and pads out the book.
I disagree strongly. The best authors can reveal depth of a character with a few well-chosen words or acts here and there and continue the revelation of that character through his or her subsequent actions and reactions.
It is also a question of what the individual reader or writer might regard as padding. An author might, for instance, in the interest of maintaining the flow of the plot, decide to spend a few less pages describing Lord Juss's throne room in order to have a few words left over to explain his inner thought processes. Actually, I am having a hard time coming up with any other writer capable of padding a story like Eddison could. There is a great deal of charm in much of the detail, but it can also reach a stage that is well past cloying.