Sir Sean Connery spent time underway aboard the U.S.S. Puffer (S.S.N. 652) preparing for his role. He was given Commander status, and allowed to give commands while underway (with the Captain beside him).
After the release of Tom Clancy's novel "The Hunt for Red October," some members of Congress contacted the CIA, demanding to know why the Russians had invented a caterpillar drive before the U.S. Navy did. Although a result of good research, the caterpillar drive is pure fiction.
When the movie was first released on VHS in 1991, the tapes were red.
During filming, several of the actors portraying U.S.S. Dallas crewmen took a cruise off the coast of San Diego on the U.S.S. Salt Lake City (S.S.N.-716) a real Los Angeles-class submarine. To help Scott Glenn train for his role as the Dallas's commander, the real commander of the Salt Lake City, Thomas Fargo, ordered his crew to treat Glenn as equal rank, first giving reports to Fargo, then give the same report to Glenn. Glenn based his performance of Captain Bart Mancuso on Commander Fargo, giving orders in a calm, even voice -- even in tense situations; saying, "whatever good happened in the performance, basically I owe to now Admiral Fargo, thank you sir."
While under attack from the other Russian submarine, Ramius asks Ryan which books he has written. Ryan describes one book about Admiral Halsey, to which Ramius replies "I know this book. Your conclusions were all wrong, Ryan. Halsey acted stupidly." No further explanation is given, but they are referring to the Battle of Leyte Gulf in October 1944. The U.S. was about to invade the Phillipines and knew the Japanese Navy would counterattack, but didn't know where. Halsey's fleet was supposed to guard the northern approaches to the invasion beaches. The Japanese Navy sent a small fleet of their ships in one direction as a decoy. Halsey took the bait and sent his entire fleet after it, but without informing the rest of the U.S. Navy of what he was up to, leaving the north completely unguarded. (This action is what Ramius refers to when he says "Halsey acted stupidly.") The rest of the Japanese Navy was able to slip through unscathed and undetected. If not for the valiant action of destroyers and escort carriers, the Japanese fleet could have devastated the U.S. invasion fleet.
John McTiernan Sr.: During Jack's briefing for Jeffrey Pelt, the director's father as one of the advisors in attendance.