When Jim wanted me to do something
As others have already said, your father was a remarkable person. Not only was he one of the heroes of structural engineering, and a founder of structural control and structural reliability, but he was one of the nicest, most sincere and most caring people I have ever met.
I remember once sitting at a conference when Jim wanted me to do something. He sat down next to me, put his arm around my shoulder, and I knew I was about to be asked something to which I would agree; you had to.