It's a Wonderful World (1939)

This is a cracking, quickly paced little comedy thriller with Claudette Colbert really on form as the rather dreamy poet who finds herself caught up in James Stewart's murder shenanigans. He is a PI whose wealthy pal is held for murder. When Stewart ("Johnson") is offered $100k to get to the bottom of it all, he tries to investigate, gets arrested, escapes from the cops into a river and alights on poor old "Edwina Corday" (Colbert) whose car he hijacks. After a bumpy start to their relationship, the pair of them are soon on the case to solve the mystery. It moves along well, the two stars - along with Guy Kibbee as "Cap Streeter" - gel well with some pithy lines from Colbert and daft scenarios well executed by them both as they close in on their prey. In the end, it doesn't really matter who killed whom - it's just an enjoyable vehicle for the two stars who are having some fun - and it is contagious.