Because of the criticism directed at producer Stanley Kramer by the American Legion for hiring Nedrick Young, whom they considered subversive, Moss Hart as president of The Authors League of America sent Kramer a telegram: "The Authors League of America council, which has always unalterably opposed any form of blacklisting of writers, unanimously voted at a meeting today to commend and applaud you for your courageous stand in rejecting publicly the effort to interfere, on pseudo-patriotic grounds, with the right of writers to work."
The real-life John Thomas Scopes upon whom the Bertram Cates character was based had an unusual epilogue of his own. After the famous trial, he was approached by a representative of the University of Chicago, which offered him a scholarship for graduate study in geology. Scopes then did geological field work in Venezuela for Gulf Oil of South America, and went on to make a name for himself in the field of geology.
In the scene where Drummond (Spencer Tracy) tells the story of his rocking horse "Golden Dancer" to Brady (Fredric March), they are sitting in rocking chairs on the porch of the boarding house. The actors are both rocking their chairs but are never in sync with each other to emphasize their differences of opinion.
In real life, Clarence Darrow asked the jury to find John Thomas Scopes guilty so that he could appeal to the Tennessee Supreme Court where he could put an even bigger dent on the law. The guilty ruling was overturned but on a technicality, which wasn't Darrow's hope.
The actual Scopes Monkey Trial testimony was quite dull, until Clarence Darrow (Drummond) called William Jennings Bryan (Brady) as a defense witness. Firing questions about the earth's origins and Adam and Eve, Darrow quickly forced Bryan into raging contradictions, proving his point that the Bible, in light of scientific knowledge, cannot be interpreted literally.
When Stanley Kramer offered the role of E.K. Hornbeck to Gene Kelly, Kelly initially turned it down. Kramer told him that his co-stars would be Fredric March and Spencer Tracy, and Kelly changed his mind. This was a risky move on Kramer's part, as he had not yet asked March or Tracy to participate.
There are several references to "chautauqua meetings" and "chautauqua tents" throughout the film. "Chautauqua" was an education movement for adults in the U.S. The movement was highly popular at the end of the 19th and in the early 20th centuries. Chautauqua meetings spread throughout rural America until the mid-1920s. William Jennings Bryan (represented in the film by "Matthew Brady") was a popular speaker at chautauquas. Chautauqua meetings or assemblies brought a variety of entertainment and culture for the whole community, with a range of speakers, musicians, etc.
When Drummond's attempt to call scientific experts to the stand to testify in behalf of the defense is thwarted, Stanley Kramer adds a couple of elements from the actual John Thomas Scopes Trial, combining the fiery closing of Clarence Darrow's speech on the motion to quash the indictment with the change in which Judge Raulston cited Darrow for contempt.
Writers Jerome Lawrence and Robert E. Lee named the overzealous prosecutor "Matthew Brady". When Roscoe 'Fatty' Arbuckle was tried for manslaughter three times in 1921 and 1922, the real overzealous prosecuting San Francisco District Attorney was named Matthew Brady. Matthew Brady was also the name of several real life historical figures including an English "Gentleman Bandit" who robbed several farms but refused to harm women in Tasmania in the 1820s. But the name Mathew Brady (with only one t) is most famously connected with the famous portrait and landscape photographer of the mid-1800s, best known for immortalizing American Civil War casualties.
The buzz about Fredric March's and Spencer Tracy's performances spread around the studio lot and Hollywood to the point that a lot of extras made their way to the set just to see the action. In one instance, the extras were applauding so much at one of March's dramatic speeches that they ruined the take by not waiting until the end.
The subplot concerning Cates' engagement to the Rev. Brown's daughter Rachel, and Brady's manipulation of the girl to give damaging testimony at the trial, is entirely fictional. The real-life John Thomas Scopes had no known fiancée or girlfriend at the time of the trial.