Oh yay, I'm glad that you care about this too :) I feel like it's particularly important when the book has taken some pains to inform us that the characters ought to be really different!

Have you read Possession (Byatt)? That was the first book I read (I think I was in high school?) where I really got that the author was specifically and stylistically making the characters sound very different, although being a teenager I expect I didn't get half of what she was doing.