At the time, Ethan Hawke had only appeared in two other movies, so I can’t blame him for trying to present himself as mature and professional so that he could make a good impression.
While Robin Williams was a versatile performer, he definitely loved to improvise on many of his projects, particularly the comedic ones.
Dead Poets Society obviously fits nicely into the drama genre, but that doesn’t mean fun couldn’t be had during the filmmaking process.
As Ethan Hawke acknowledged during his appearance on The Graham Norton Show, Williams was incredibly funny, relaxed and inventive, and whenever he made the crew laugh, the more encouraged he became.