Steven Spielberg was originally set to direct. He later recommended Martin Scorsese for the job and personally called the director, letting him know that this was a commercial film that had potential to be a hit, which would exercise more power for Scorcese to make his films.
The climactic scene out in the swamp was filmed in John U. Lloyd State Park, in the middle of a mangrove swamp. A tropical depression set over the set for four days, so the film crew had to wait for the storm to stop, so that they could make their own rain.
Both Robert De Niro and Nick Nolte had to alter their physiques for the film because the 6'1", bulky, Nolte is clearly larger than the 5'10", slimmer De Niro. Nolte lost a good deal of weight while shooting the film and De Niro bulked up his muscles considerably until De Niro was clearly Nolte's physical superior. Interestingly the original Sam Bowden (Gregory Peck) was also slightly taller than the original Max Cady (Robert Mitchum).
De Niro's Cady accent came from an earlier role where he played a southerner. To prepare for the role, De Niro took excerpts of the script and a tape recorder into southern towns and would ask locals to read the lines into the tape.
In the original script, Leigh only met Max Cady at the end. Jessica Lange suggested the scene where they talk outside her house be added to the script, because she felt there should be a meeting between the two before the climax.
Martin Scorsese wanted the Bowden's house surrounded by oak trees, covered by hanging tufts of Spanish moss. He wanted it so it looked like a sunny oasis by day, and isolating by night. Therefore, it would make a perfect dark cover for Cady.
Lori, talking with Max, says, "Now weren't I the bozo on this bus!" - a reference to the 1971 album by The Firesign Theatre called "I Think We're All Bozos on this Bus". A reference like this may seem against period, but Martin Scorsese, like the Firesign Theatre players, was part of the counter-culture scene of the '60s and early '70s. Considered with other elements in the script, such as the lax attitude toward marijuana, this doesn't seem so out of character.
During the opening sequence Max Cady is seen working out in his cell and the camera pans over his jail time reading material. One of the books featured is The Cell Within by Jake Manning. This is not a published work and only exists as part of a Miami Vice storyline _Miami Vice: Season 5, Episode 13_ in which Tubbs is tormented and imprisoned by the author Jake Manning, an ex-con he helped convict years before.