One of the most difficult things, in my opinion, while writing a story is to give every character a distinctive voice. Often times all the characters sound almost the same. I thought that in "Shriek: An Afterword" (a wonderful book that I cherish) you create a fantastic setting to differenciate Janice and Ducan. In any given story, how do you make sure each character has a trademark voice?
Sometimes it's easy to do just by having one in third person and one in first person. Sometimes as with Shriek it's just that hopefully you know the characters well enough that the voices are different. But also in Shriek because they are brother and sister you expect some similarities, which helped me out a bit. And thanks for the kind comment about Shriek--still the hardest book to write technique-wise and still my own personal favorite.