If you’re familiar with Schindler’s List and Steven Spielberg (or even just have a vague knowledge of common Jewish last names) then you can understand why doing the movie was a difficult experience for him.
If not, then it’s worth knowing that Steven Spielberg is a highly famous and influential Jewish filmmaker, and Schindler’s List is a 1993 film based on the story of Oskar Schindler, a German businessman who saved the lives of over a thousand refugees during the Holocaust.
The shooting of the film was very emotional for Spielberg due to his roots and the antisemitism he faced as a child. There were some scenes in the film that, heavily based in reality, were so dark that he couldn’t watch while they were filmed. So dreary were things that Robin Williams took to calling Spielberg every two weeks to lift his spirits with various jokes, mainly because there wasn’t much humor on the set.
On the subject of the filming, Spielberg later said “I was hit in the face with my personal life. My upbringing. My Jewishness... And Jewish life came pouring back into my heart. I cried all the time."