That sounds like something he said to Bogdanovich, I forget whether in the print version or on the tapes. I seem to remember him saying he has to keep shooting or the film dies. Other directors liken that to a train chugging along, that they have to keep throwing track in front of it to keep it going, or it will wreck and the film will die. Still other directors say this is complete nonsense.
The quote I want to find, which I believe is buried in one of Welles' Harry Lime radio scripts, is where Welles stated explicitly what the moon symbolized for him. In middle age onwards he was completely evasive about it, saying the moon was very important to him yet refusing to explain it at all. Damn that hole in my memory that the answer fell through, for I had found it at one point.