trivia bull

It was a combination of a great script and the actors' improvisation; for example Bogart came up with that little bit with Bacall walking around him to see if any strings were attached, and then that cute little slap she gives him, telling him to shave. The most famous line of the film ("You know how to whistle, don't you Steve? You just put your lips together and...blow.") wasn't improvised, however; Howard Hawks wrote a small scene for Bacall to read in her audition, and when it became so successful he had to work it into the script.

I just found this in Bogart's biography:

Dan Seymour described how "we'd sit down every morning, Marcel Dalio, Bogie, Betty, everyone. Hawks would say, for instance, 'You know the essence of the story, Bogie?' and Bogart would say, 'Yeah, I know the essence. It stinks.' So Hawks would say, 'Bogie, you say so-and-so to Dan.' Then he'd turn to me: 'What are you going to say to him?' I'd say, 'What the hell do you want me to say?' And he'd say, 'Answer him. Do what the character would do, that's all.'

They had a secretary writing it all down, and we'd do it agin, and rehearse it. Then Hawkds would look at Faulkner and ask, 'What do you think?' And Faulkner would reply, 'I think it's all right. It'll play, it'll play.' And that's how the whole show was done."