Dustin Hoffman planned the moment when he throws his wine glass against the wall during the restaurant scene with Meryl Streep. The only person he warned in advance was the cameraman, to make sure that it got in the shot. Streep's shocked reaction was real, but she stayed in character long enough for writer and director Robert Benton to yell cut. In the documentary on the DVD, she recalls yelling at Hoffman as soon as the shot was over for scaring her so badly.
The famous ice cream scene, where Billy challenges his father by skipping dinner and going straight for dessert, was completely improvised by Dustin Hoffman and Justin Henry. Writer and director Robert Benton liked the scene so much that he decided to keep it in the film.
The final courtroom scene had one important rewrite: Joanna's explanation of why she left. Robert Benton feared major delays but, in fact, Meryl Streep had in mind what she wanted, and quickly rewrote the monologue. Benton said, "Well, the scene is brilliant. I cut only two lines. What you see there is hers."
Meryl Streep wrote her own courtroom speech upon writer and director Robert Benton's suggestion, after she told him she wasn't satisfied with the way it was originally written.
The strength of the performances of the two leads can be at least partly attributed to what was going on in their private lives at the time. Dustin Hoffman was in the midst of a messy divorce, while Meryl Streep was still recovering from the death of her lover, John Cazale.