Mr. & Mrs. Smith (1941)

A couple married for three years is in the aftermath of a huge fight when Mr. tells Mrs. he'd have stayed single if he had to do it all over again. When they find out that, due to a jurisdiction technicality, they're not legally married, he doesn't rush to make an honest woman out of her. She is not amused and shenanigans ensue.

It's fun and fast, but the alternate love interest is just not enough of anything to bother with. He's gentlemanly and dull where I'd have preferred an extreme of any type (boorish, highly sexualized, dumb as a rock -- anything). Instead the competition is perfectly fine and that made for very little tension about what might happen.

B