Nicholson pulls off the greatest ad lib of all time, when he hacks
through a bathroom door with an axe and screams, "Herrrrreee's
Johnny!" According to the IMDB,
set designers originally had a prop door that would be easier to hack
through, but Nicholson became so good at swinging the axe, that the
fake one would shatter to bits after a couple of hits, so a real one
was finally used instead.
Nicholson's ability to get into the character and add things on his
own have helped make Jack Torrance one of the all time great movie
psychos. The scene where he's screwing around and throwing a tennis
ball over and over again instead of writing is a great example of
this, as the script merely said, "Jack's not working". Nicholson
improvised the whole thing.
course, when you've got Shelly Duvall screaming in your ear, it's
pretty easy to get the inspiration to be murderous.