Martin Scorsese could not figure out a suitable ending for the film. He asked Brian De Palma, Steven Spielberg, and Terry Gilliam to watch the film so they could give him their opinion on how the film should end.
Joseph Minion's script was his thesis for Columbia Film School. He got an "A" from his teacher, Yugoslavian director Dusan Makavejev.
Martin Scorsese told Griffin Dunne to refrain from sex and sleep during filming in order to get a more realistic feeling of paranoia.
It would have taken Paul approximately 1 hour 47 minutes to walk home---assuming he walks at an average speed of three miles/hour (one mile every 20 minutes) and hits all the cross streets at the correct time to cross (The distance from Paul's uptown apartment ("East 91st Street") to Kiki's SoHo loft ("28 Howard Street, near the corner of Crosby") is approximately 5.3 miles) [Google Maps].