He has a blog describing his experiences playing for ballet, opera, chamber music, etc. Here is an excerpt about an ABT tour in 1964:
I played for the first time Billy the Kid by Aaron Copland. There were many tricky and totally exposed cello entrances that came all on different beats, and playing by ear, which was my modus operandi, was not working very well. Bill Brohn, conducting and seeing the terror in my eyes, threw every entrance right at me. On a third beat, on a second beat, on a third beat, and on each one said, as loud as he could, muttering under his breath, “Learn to count! Learn to count! Learn to count!” Since, at that point, I knew the jig was up, I approached Walter Hagen and confessed my orchestral inadequacies and how horrified I was of the fast 7/4 time coming up in a ballet called Miss Julie, based on the Strindberg play. Walter did something for me that was the epitome of kindness, and which I always appreciated, never forgot, and was able, many years later, to repay his kindness.
During every bus trip for the next week, Walter would spend time with me going through the parts, solfeging all the difficult rhythms, until I simply knew how they went. Also, he would conduct while I sang from dictation what he had taught me, so that I could put it together with his stick.