Laurel & Hardy could very easily have slid into musical comedies, as could the Marx Brothers. Hardy had a tenor voice, put to good use in WAY OUT WEST, and Stan (while he was not as good a singer) could talk sing quite nicely (and was prepared to do comic singing - his change of voice in singing "In the Blue Ridge Mountains of Virginia" in WAY OUT WEST is an example). As a result a musical number will frequently pop up in their movies. In SONS OF THE DESERT it's the HONALULU, BABY! number. In SWISS MISS there were several (even a ditty, supposedly composed by Walter Woolf King, entitled "Crick, crick, crick goes the cricket"). Even in their earliest features numbers appear (in PARDON US we hear "MICHIGAN"). So it was not unexpected that Hal Roach and Stan Laurel would do full scale musicals. They turned to operettas (or spoofs of operas like FRA DIAVOLO) because the costumes and settings gave opportunities for Stan to come up with new pieces of business for himself and Babe Hardy.
The actual opera by Auber is more dramatic than this comedy. Fra Diavola dies at the end (he is, after all, a villain). But here there is a light hearted element that overcomes the original. Stan and Ollie (or Stanlio and Ollio) are robbed by brigands on the road and decide to turn brigand themselves. Naturally, Ollie decides that he will protect them from discovery by claiming to be the infamous Fra Diavalo. Their initial attempt at theft is hardly successful. They confront a man with a hard luck story, and end up giving him money. Then they seem to be more successful confronting a younger man, until Ollie brags that he is Fra Diavolo. The younger man demands proof. Diavolo always sings a theme song, and everyone knows his voice. Ollie starts singing the tune, and the man (you've guessed it - it's Dennis King) continues singing it. They almost get hanged for that, but King decides to use them as minions in a plot to rob an English mi-lord and his wife (James Finlayson and Thelma Todd*).
(*Finlayson's name, as a joke, is Lord Rocburg. In reality, the character in the opera was Lord Cockburn.)
The bulk of the film deals with King and the boys in the inn run by Henry Armetta, where Finlayson and Todd are residing on their trip. King is romancing the frivolous and bored Todd, and hoping to get her jewelry. Finlayson is suspicious of her activities, but is not swift enough to catch King in action (at one point, he is mistaken for King by Ollie and Stan, who lock him up after beating him, and start telling him off - they think of course he's King, who listens to them annoyed but amused).
The music is actually not overdone, and King (who had a fine trained voice) gets several opportunities to sing. He was not the first major Broadway star to work with the boys (Lawrence Tibbett had in ROGUE SONG) nor the last headliner to do so (Dante the Magician would in A-HAUNTING WE WILL GO). But he seems to work quite nicely with them, sharing screen time, and even showing elements of comic timing and reactions. In one song number, he even shudders and turns away from an ugly woman while addressing a romantic passage in the tune.
For an early example of their use of operetta, THE DEVIL'S BROTHER is (as Stan Laurel said) one of their best films. Of course, to most people, it will always be recalled as the film where Stan drives both Ollie and Henry Armetta to distraction with his "finger wiggle" and his "earsie - eyesie - nosie" games that he can handle with ease but the other two can't quite coordinate. It is fun to watch here, and would later be subject to a rare repeat comment: in BABES IN TOYLAND, when Ollie insists that anything Stan can do Ollie can, Stan smiles and shakes his head. He then does "earsie - eyesie - nosie", much to Ollie's annoyance.
