James Cagney was a terrific dancer, but he was one of the earliest and perhaps most (in)famous practitioner of the dubious art of talk-singing.
Like other film actors who carried on the tradition such as Rex Harrison in "My Fair Lady," Cagney couldn't hold a sustained note. (Or, on the rare occasions when he tries to, like with the title song, his voice is noticeably weak.)
The solution was to have him do a sort of melodic form of speaking that emulates the rhythm and tone of singing, but without any sort of musical lilt.
I suppose that's an acceptable solution if he just had a song or two in "Yankee Doodle Dandy," but it's a biopic of one of the greatest song-and-dance men Broadway ever saw. So to have him constantly standing there doing that strange vocal cadence while everyone else in the movie is singing their hearts out gets a little old after awhile.
The obvious alternative would have been to have another actor dub Cagney's singing. But he had such a distinctive voice -- a hard, close-throated moan with elongated vowels -- that it was likely impossible to find someone who could sing while still sounding like Jimmy Cagney.
This doesn't detract from his dancing, which is spectacular. My favorite bit is when he is playing an old man on stage, and a fan (who would later become his wife) thinks he really is old, and is surprised when he does a frenetic tap number that would have left a man of half of his presumed years in a puddle.
Cagney plays George M. Cohan, one of the early giants of Broadway, who wrote a number of songs that still linger today: "Over There," "Give My Regards to Broadway," "The Yankee Doodle Boy," "You're a Grand Old Flag" and "Mary Is a Grand Old Name."
The movie starts with him as a child star in the family act, The Four Cohans, with his parents (Walter Huston and Rosemary DeCamp) and sister Rosie (Cagney's real-life sibling Jeanne). Through spunk and grit, he manages to get his own show staged along with longtime partner Sam Harris (Richard Whorf) and eventually becomes the biggest name on Broadway.
The framing story is rather cutesy: After opening his big return act portraying Franklin D. Roosevelt, Cohan is summoned by the president to a night meeting at the White House. Figuring he's going to get chewed out by the chief executive, he instead tells FDR his life story. At the end he's given the Medal of Honor by a special act of Congress. (It's unclear if the film meant the Congressional Gold Medal; the Medal of Honor is strictly for military heroism.)
The story is built around the various songs and shows that highlighted Cohan's life, along with his courting and marriage of Mary (Joan Leslie). It's a rousing good time, with Cagney giving a memorable (when not talk-singing) performance as the cocky but good-hearted Irishman. Cohan's shows were noted for their comedy and unabashed patriotism
"Yankee Doodle Dandy" was directed by Michael Curtiz, one of the great Golden Age directors who doesn't get the recognition of other giants of that era. In that same year, 1942, he also directed Cagney as a fighter pilot in "Captains of the Clouds," and a little picture called "Casablanca."
In this era, when filmmakers like James Cameron or Terrence Malick can go a decade or more between feature films, the prolificacy and quality of work by someone like Curtiz is astonishing.