The scenes with Oliver Barrett walking alone through a snowy New York were added after principal photography was completed. The production was almost out of money and did not have the necessary funds for permits to shoot in New York City again - so all the shots were grabbed illegally using a skeleton film crew and Ryan O'Neal.
In 1972, this movie was the most viewed film on television of all time.
Author Erich Segal wrote the screenplay first, then adapted this into a novel, which was published before the movie's release, and became a runaway bestseller.
To prepare for their roles, Ryan O'Neal learned to ice skate, and Ali MacGraw learned to play the harpsichord. When Arthur Hiller settled on Ryan O'Neal, he rehearsed the actor and Ali MacGraw for 10 days, because he wanted them to be friends before filming began.
It was the production of this movie that led Harvard University to prohibit almost all commercial filming on their campus, reportedly because of disruptions to students and damage to the campus, including fake snow killing several trees. The production was kicked off the campus after only a week of filming. Most subsequent movies set at Harvard University filmed at other nearby schools.