Atom Egoyan intended to use his usual stable of actors for The Sweet Hereafter, so Bruce Greenwood seems to have been in on this film from its first conception. Atom does offer some insight into the casting of Bruce as Billy though, with the following statement:

Egoyan went on to explain, the casting of Bruce Greenwood was not so simple.

"Bruce, I originally cast as Sam, [Nicole]'s father [who ended up being played by Tom McCamus] and Bruce said that he would rather play Billy, that Billy was the person he really identified with. So I had to step down from playing the role of Billy myself."
University of Toronto Varsity Review 10/20/97