During filming, tourists were still allowed onto the island, and a new boatload would arrive every half-hour. They became so much of a distraction that the majority of filming was moved to night shoots.
The windshield wipers on the two front windows on the boat at the very beginning of the movie were installed specifically for that scene. Originally, the two front windows had bars going vertically down the center, similar to the numerous other windows surrounding the pilothouse.
The guard to whom Doc shows off his "accident" in the carpentry shop is named Mr. Zimmerman. "Zimmerman" is German for "carpenter".
The dangerous escape down the prison wall and into the water was performed without stunt doubles. Director 'Don Siegel (I)' twice thought that he had lost his actors to the treacherous currents.
Fifteen miles of cable were required to reconnect the island to the city's electricity, and a great deal of work was required to restore the prison to its 1963 state. Many of the improvements were kept intact after the film.
Visitors to Alcatraz Island may be interested in viewing the actual cells that had been home to the real-life inmates depicted here by Clint Eastwood (Frank Morris), Fred Ward (John William Anglin), and Jack Thibeau (John's brother Clarence Anglin). Their cells #138, #140 and #144 are located in B Block ("Michigan Avenue") along the bottom row. The cells used in the film, however, are located in the C Block, middle cells on the Broadway side along the bottom row.
The Warden, who is never named during the film, at one point refers to his "predecessors, Wardens Johnston and Blackwell." Warden Blackwell was the actual warden at the time of Frank Morris's escape, and this script reference was clearly done to avoid legal trouble.
Fresh water had to be hauled in by boat to create the rain in the opening scene. Using saltwater would have damaged expensive equipment.
The boat bringing Morris to Alcatraz in the very beginning of the movie (M/V Warden Johnston, named after the first warden of Alcatraz) was actually used to transport prisoners to and from Alcatraz and was not a mock-up. The boat was built by prisoners in McNeil Island in Washington for this specific purpose, and was used for much of the time Alcatraz was in service as a prison. The makers of the movie borrowed the Johnston from the Sea Scout crew who was operating her at the time (Sea Scout Ship #145, based in Redwood City, CA). Gary F. Warren, who is listed in the credits as a guard, was in fact the boat operator, and was also the leader of the Scout crew at the time.
The television show, "MythBusters" (2003) proved that this escape worked (or was at least plausible). They recreated the entire escape right down to using the same materials to which the cons had access. They even used the same type of raincoats from which the boat was made. They successfully crossed the bay and reached the shore in exactly the spot that the cons were supposed to have reached.