One scene involving Robert Redford on the phone is a continuous six-minute single take with the camera tracking in slowly. Towards the end, Redford accidentally calls the phone caller by the wrong name. But as he stays in character, it appears genuine, so the take was used in the final cut.
The two lead actors memorized each other's lines so that they could interrupt each other in character. This unsettled a lot of the actors they were playing opposite, leading to a greater sense of authenticity.
Hal Holbrook was the first (and only) choice to play the shadowy informant Deep Throat. During pre-production of the casting process, Bob Woodward -- while looking at various actors' head shots and resumes, but not revealing Deep Throat's true identity (being the former Deputy Director of the FBI, Mark Felt) -- insisted to director Alan J. Pakula that Holbrook was the best choice to play Deep Throat. (Holbrook, in fact, bears a strong resemblance to Mark Felt.) Holbrook originally turned the role down, thinking it would not be a significant part. However, Robert Redford came to Holbrook's house and convinced him to take the role, saying that Deep Throat would be the character that the audience would remember more than any other in the film.
The film introduced the catchphrase "follow the money", which was absent from the book or any documentation of Watergate.
On Tuesday, May 31, 2005, in advance of a revelatory July 2005 "Vanity Fair" article written by his attorney and spokesman, 91-year-old Mark Felt acknowledged publicly for the first time that he was "Deep Throat," a fact corroborated by Bob Woodward and The Washington Post. At the time of the Watergate break-in, Mr. Felt was the Deputy Director of the FBI, the second-in-command.