Just before John Coffey is about to be executed, he's sitting on his chair in the cell. As he stands up, you see him lift his leg onto a platform. The platform was used to make him look about a foot taller than he actually is.
In the movie John Coffey has sort of a "messiah"-like role. His initials J.C might be a reference to Jesus Christ, as this is very fitting for his role. [This is a not-so-subtle attempt which places Paul Edgecomb in the role of Pontius Pilate]