Apparently much of Diane Lane's 'electric performance' was cut due to running time constraints. At a press conference director Peter Landesman and Liam Neeson both championed Lane's performance saying how devastated they all were (especially Lane herself) that so much of her superb performance was left on the cutting room floor. There were hints that these scenes may be included as 'deleted scenes' or as an 'extended cut' on the home video release of the film.
Mark Felt did not choose Bob Woodward at random from the Washington Post's roster of reporters. Felt and Woodward had known each other for a few years with the two having initially met one another while Woodward was serving in the U.S. Navy as an Admiral's aide. In fact Woodward had sought out Felt's advice on his future when his discharge from the Navy was approaching.
This is the second time Peter Landesman has worked with either Scott Free or Playtone productions. The other Playtone movie Landesman directed was Parkland (2013) and the other Scott Free movie was Concussion (2015).
The trivia item below may give away important plot points.
Although it is not mentioned in the closing credits one of the main reasons Felt 'outed' himself as Deep Throat was that he was in the early stages of dementia (Felt was 91 at the time) and wanted to discuss the matter while he was still able to recall the details of the Watergate scandal.