The Bachelor and the Bobbysoxer (1947)

The title of this movie was changed to "Bachelor Knight" in the U.K. which doesn't really say what the films is all about; like it says above, this is the story of a carefree bachelor (Cary Grant) who unwittingly attracts a besotted teenager (Shirley Temple in one of her few late roles).  At the threat of being charged with unseemly conduct, her sister judge orders him to date Temple until she comes to her senses.  Depending on your point of view, it is either easy-going froth played by able comic actors (other than Temple) or a complete embarrassment as Grant is forced to act like teenager too.  The fact that he is able to carry off such silliness is a testimony to his acting chops. The rest of the cast which includes Myrna Loy, Rudy Vallee (once a singing sensation), Harry Davenport and Ray Collins add to the general amusement.