Al Shean (born Adolph Schoenberg), who plays the Professor in the film, was half of one of the most popular teams in vaudeville--Gallagher and Shean. He was also the younger brother of Minnie Marx, the matriarch of The Marx Brothers clan, and was instrumental in writing many of the sketches that his madcap nephews first performed on the vaudeville circuit before their enormous success on Broadway and in Hollywood.
The comment that Spencer Tracy makes about the "Rooney kid" is an ad-lib (watch Jeanette MacDonald's expression reacting to it). Tracy had worked with Mickey Rooney earlier that year in Riffraff (1936) and knew that director W.S. Van Dyke abhorred retakes, priding himself on bringing in productions fast and under budget--hence his nickname, "One-Take Woody".
Jeanette MacDonald's older sister, Blossom Rock, signed with MGM and was given the name Marie Blake. Jeanette's character in this film was named Mary Blake. Her sister used the name Blossom Rock when she played Grandma Addams on The Addams Family (1964).
Jeanette MacDonald brought the screenplay by Anita Loos to the attention of MGM head Irving Thalberg with the express idea that she should headline alongside Clark Gable. Thalberg readily agreed although Gable did not get along with MacDonald during filming. He objected to her singing at him and would eat garlic before their kissing scenes just to annoy her.
Spencer Tracy, playing a priest, makes a note to himself in one scene, "That Rooney kid skipped Mass again . . . " Two years later he again plays a priest in Boys Town (1938) who is tasked with reforming a boy played by Mickey Rooney.
Clark Gable and Spencer Tracy would go on to make two other films together, Test Pilot (1938) and Boom Town (1940). By the end of the filming of the latter, Tracy had tired of always receiving supporting billing below Gable and insisted on shared billing going forward. Gable did not agree with this and the two never worked together again.
The character of Blackie Norton was inspired by Wilson Mizner, a fellow writer of Robert E. Hopkins and Anita Loos, who had worked on Broadway and at Warner Brothers and had died several years earlier, He was a notorious huckster, con artist and womanizer, with connections in gambling and underworld circles.
Spencer Tracy initially had qualms about appearing in the film because he was unsure about playing a priest. A devout Catholic, Tracy felt that he might be betraying his faith by trying to impersonate a priest for the movies. Ironically, two years later Tracy would win an Oscar for playing a priest in Boys Town (1938).
While writers Anita Loos and Robert E. Hopkins considered W.S. Van Dyke a director of considerable talent, they became worried shortly after filming had begun. "Van Dyke," said Loos, was "an oaf when it came to the subtleties of the San Francisco tenderloin. We were horrified watching Woody direct a scene where Blackie reproves an underworld sweetheart for wearing a gaudy necklace and, indicating it, said, 'Blackie told you not to wear that. It looks cheap.' Those words should have been tossed off gently and with a smile, as Wilson Mizner would have done. But Van Dyke caused our hero to jerk the necklace off the girl's throat with a brutality that cut into her skin and to bark out the dialogue in the manner of a hooligan. Not all of Gable's native charm could overcome the loutish behavior in which Van Dyke was directing him. We proceeded to [producer Bernard H. Hyman's] office to demand a retake. Bernie was surprised. 'Why, I thought the way Woody directed that scene was swell!' For over an hour Hoppy and I conjured up the spirit of [Irving Thalberg], explaining that one crass move on the part of our hero would cause the entire movie to flounder beyond recall. Bernie, bless his simple heart, finally got our viewpoint. He ordered the sequence reshot with Hoppy on the set to guide Van Dyke. Pacing the Alley the next day I said to Hoppy, 'When Irving died, he'd taken the studio to the top of a toboggan run. From now on there's only one direction MGM can go.' 'Babe, you just said a mouthful!,' Hoppy declared, thus repeating a phrase that he himself might have added to the English language."
Clark Gable got along with Spencer Tracy. They were close in age, both liked to tie one on, and the two managed to forge a friendship. Both possessed qualities that the other admired. Gable deeply respected Tracy's acting ability, and Tracy couldn't help but be envious of Gable's heartthrob status as a leading man.
Before filming his first love scene with Jeanette MacDonald--whom he did not like and did not enjoy working with--Clark Gable reportedly filled up on a big spaghetti lunch. When the time came for him to kiss MacDonald, his breath was so bad from garlic that she nearly fainted.