I've always thought:

Bill Paxton as Walter

Cate Blanchett always struck me as more of a Snowbird than a Heather

Though he's getting a bit old, Russell Means would have been perfect for Shaman back in the day; as it is, Adam Beach could probably fit in the role.

Mac is a tough one; Vince Vaughn is too slick for the part, much as I love VV. I can see Josh Hartnett in this role, though, again, he's a bit young.