While Judd Apatow is a talented writer, it’s no secret that he is a massive fan of improvisation.
There is much less specific emphasis put on the details of his scripts – which is why Bill Burr, during the making of the recent The King Of Staten Island, felt comfortable changing up his methodology during the making of the film, and within a couple weeks on set found himself taking a different approach than normal to the screenplay.
What’s put on paper is important for structure, plot, and character arcs in his movies, but he is also a big believer in on-set magic, and in the production phase of all of his projects he has actors make up material on the day and is known to frequently throw out ideas from behind the camera.