Almost at the end of the film, Poncelet is marched to the death chamber, with Prejean's hand on his shoulder. They are led past an open door - which leads to the viewing room, where the witnesses will congregate. Some of them are already in the room. This is nonsense - it is inviting a bereaved relative to attack the condemned man. In fact the corridor to the death chamber goes nowhere near the witnesses' area, and Prejean would have had to have been escorted back through two guarded doors to reach it.
The movie is loosely based on Sister Prejean's experiences with two real death row inmates, Robert Lee Willie and Elmo Patrick Sonnier. Both men were electrocuted. In the movie, however, Sean Penn's fictional character was killed by lethal injection--because the image of a man strapped down to be injected allows for all that Christ symbolism.