Mike Moore, the Attorney General of Mississippi, played himself for the scenes involving the lawsuit.
While he was promoting The Loudest Voice (2019) on the Howard Stern show, Russell Crowe revealed that he received a book from the Marlon Brando estate, written by poet Patrick Kavanagh with an inscription to Brando by Jack Nicholson. Crowe had been told that Brando was a big fan of this movie because of his performance and watched it repeatedly. Brando had instructed his caretaker that when he died, he wanted Crowe to receive the book. Brando has also worked with Crowe's co-star Al Pacino in The Godfather (1972).
After filming a scene shot at the school, Russell Crowe pranked young castmates by screaming and ripping out his hair. The realistic gray wig had many fooled and horrified, until the crew and Crowe erupted in laughter.
The courtroom where Jeffrey Wigand (Russell Crowe) gave his deposition was not a set. The filmmakers used the actual courtroom in Jackson County, Mississippi where the real Wigand's deposition was given.
Jeffrey Wigand, the anti-smoking subject of this movie, requested a ban on cigarettes in the movie. However, cigarettes are smoked in the movie at least three times: (1) by a woman in the background as Wigand (Russell Crowe) enters the airport, shortly before being served with a restraining order; (2) by a Muslim soldier seen briefly while Lowell Bergman (Al Pacino) is being transported to the Hezbollah meeting site; and (3) by a photographer with whom Bergman converses briefly about what might be going on inside the courtroom.