Marilyn Monroe had no confidence in herself. She found it very difficult to concentrate, and she really didn't think she was as good as she was. She'd worry about all kinds of things, and she would do the very difficult things very well. Sometimes she was very distracted and couldn't sustain it, and you had to do it in bits and pieces; sometimes she was in such state of nerves that you'd have to shoot individual lines. But such was her magic that you'd put them all together and they seemed as though she spoke them all at one time. She was a real movie personality, a real movie queen.
George Cukor in a 1969 interview with Peter Bogdanovich (published in "Who the Devil Made It," 1997)