One thing about Bing Crosby as a film performer is that while he didn't make the very best musicals, he was probably the very best at a particular kind of number that everybody tried to do. I'm talking about the "clowning around" number, where the leads act all goofy, mug a lot and do silly business while singing a silly song. Crosby has one of these in almost every movie he did, and he's very good at them. He was good at making lines and business seem spontaneous, even if they weren't, and at getting a "live" feeling into the very canned process of shooting a movie musical number.
A lot of performers had a fondness for this type of number but rarely did it well; Gene Kelly is an example of a guy who always had to have a three-minute mugging/clowning scene that almost always is a low point in the movie (like "By Strauss" in An American In Paris -- though the "Moses Supposes" number in Singin' in the Rain actually does work). The Rat Pack types always tried to do this kind of thing in their movies, and it was usually not very good -- but Sinatra did pull it off when he clowned through "Well, Did You Evah!" with Crosby in High Society. Bing was just good at this kind of thing.
Here he is with Jane Wyman in Just For You, in the movie's big number "Zing a Little Zong." The song and the number are obviously an attempt to repeat the success of the Crosby-Wyman duet "In the Cool, Cool, Cool of the Evening" from Here Comes the Groom, but Crosby and Wyman seem like they're having fun, and the tune is another one of those insidious Harry Warren tunes that you can never get out of your brain.
Incidentally, Jane Wyman is an example of how little a movie actor is helped by being able to sing. In the theatre, a good actor who can also sing is extremely valuable and has an expanded range of options, because it means that he or she can do straight plays and also do musicals. But movie musicals were for the most part built around specifically musical performers, who didn't usually do non-musical films. So it was pretty rare for someone to go back and forth between musicals and non-musicals, as they would in New York.
This meant that once a musical performer's non-musical career started to take off, he or she would start to phase out musicals almost entirely (Irene Dunne, for example, made very few musicals after the mid-to-late '30s). So though Jane Wyman could sing, she didn't usually; she built herself up to a star in non-musical films, then did a few musicals in the early '50s, and then went straight back into doing exclusively non-musicals. Strict separation.