Inherit the Wind (1960), produced and directed by Stanley Kramer.
I've been warned many times not to take this film as a documentary of the actual Scopes "Monkey" Trial of 1925. The play fictionalized the original events as a metaphor for the McCarthy period, in an attempt to champion intellectual freedom.
The background here is the breakdown of the humorless old time religion and its defeat by lively and good-natured intellectuals.
The authors changed the names but we know who is who:
Fredric March is William Jennings Bryan, prairie populist, orator, and three-time Democratic nominee for the presidency. He's played as a clown here: pious and small-minded. (Historical note: Bryan died in his sleep five days after the end of the trial).
Spencer Tracy is defense lawyer Clarence Darrow. Skeptical and humanistic, this and the Leopold and Loeb trial were his most famous cases. The latter crime was adapted by Hitchcock for Rope (1948) and Richard Fleischer in Compulsion (1959).
Gene Kelly is witty, sardonic journalist H. L. Mencken, who first called it the "Monkey Trial". Note that Stanley Kramer also gave Fred Astaire a dramatic role in On the Beach (1959). Elmer Gantry (1960) had another character reminiscent of Mencken, and the book was dedicated to him.
The histrionic courtroom blustering is a bit much for me. The performances are better when the old friends sit on the porch and rock together, talking over old times. You can be close to someone but still divided by issues and ideals.
The temperature in the courtroom was said to be 97F. It looks about that hot in the film, the way most people are sweating. Here is a photo of the two actual lawyers during the trial:
I recall a history -- was it in Bruce Bawer's Stealing Jesus? -- claiming that the Christian Fundamentalists, humiliated on a national stage by the Scopes Trial, went underground for several decades and built their own colleges, reemerging as a more potent force in the 1970s and 80s.
Available on Blu-ray from Twilight Time. Superb image.