Recently-widowed Richard Basehart and Valentina Cortese fell in love while making The House on Telegraph Hill (1951) and were married within a year. He moved to Italy to live with her, pretty much giving up his Hollywood career in the process. While he did make a few American movies in the 1950s, he made more European films including Federico Fellini's La Strada (1954). When he and Cortesa divorced in 1960, he returned to the States, remarried, and resumed acting in American movies and television. Cortesa pursued her career in Italy very successfully on stage and screen, and died in Milan, Italy in 2019 at age 96 of an undisclosed cause.
Julius Castle, a restaurant with a castle-like exterior located on San Francisco's Telegraph Hill, was used as the exterior of the house in the film. The filmmakers built a mansion-like exterior around parts of the restaurant to hide certain elements (such as the "Julius Castle" sign on the outside wall). Built in 1922, Julius Castle served as a high-class restaurant until it closed in 2008. (It is currently for sale.)
The grocery store where Tony worked was an actual store called Speedy's. It was open 94 years from 1914 to 2008 at 301 Union Street San Francisco, CA 94133. Google Maps Streetview shows that the corner store building still looks similar to the movie.
The role of the real Karin Dernakova in the concentration camp was played by Natasha Lytess. Just a few years later, she was employed by Marilyn Monroe as her first on- and off-set drama coach. Eventually she was replaced by Actors Studio guru Lee Strasberg and his wife Paula Strasberg, who served as on-set coaches during Monroe's last six films.