Stallone as John Book
The trivia section says that Sylvester Stallone was an early pick to play John Book. How do you feel he would have done?
I think Stallone would have portrayed the grizzled, hardened, rough Philadelphia cop with precision; he might have done better than Ford in this regard.
However, I have a difficult time seeing Stallone gracefully blend with the Amish community, as Ford did. Specifically, Stallone is too muscular, which disaccords with the Amish's austere lifestyle; and his face screams Italian (whereas Ford looks more Dutch). Therefore, Stallone as Book would have made the Amish feel like freaks or outsiders, and I think the movie specifically aimed to avoid that feeling.
Overall, I think Harrison Ford was the right guy for this role.