As a contract player at Universal-International in the 1950s, he attended Universal's acting classes with a fellow Universal contractee, Clint Eastwood. The two became friends, and Eastwood is still friends with Janssen's widow, Dani.
He was a top track star at school.
A close friend of Richard Harris, who was so upset by Janssen's death that he sat outside a cathedral in New York for hours in the snow upon hearing the news.
He has a star on the Hollywood Walk of Fame, only about a block away from the Chinese Theater. When it was placed there, it was in front of David's favorite ice cream shops as a child. The star dedication was on his mother's birthday in 1989.
Made 21 TV movies, which includes two mini-series.