Actually, Day and Cagney are constantly fighting in the movie. In fact, Day never actually displays any affection for Cagney, making it perplexing that the movie skips past their marriage--it's revealed by a newspaper headline in a scene following one where it's suggested Snyder violently raped Etting. Meanwhile, Day showers a loving smile on dull as dirt Cameron Mitchell as piano player Johnny Alderman. Well, not only is he dull, but also obnoxious, continuingly telling Etting how she feels and getting into a snit when she doesn't actually feel that way. And yet it's clear the movie wants us to want them to get together in the end.
But the movie has some good things. There are several full length musical numbers of Day singing songs Etting popularised in the 20s and 30s. And Cagney's performance as Snyder easily makes him the most interesting character in the movie. It's broadly written, and really just a stereotypical blowhard gangster, but Cagney invests in it totally, and perhaps the resonance of Cagney's many great gangster roles of the 30s helped, too. He comes out seeming like the only real person in a movie full of puppets.