To state what should be obvious, I never knew Jimmy Stewart — I was 14 when he died in July 1997 — but I sometimes feel as though I talked to him from time to time. And, in a way, I did: For the last 18 years of his life, I was a friend of director Peter Bogdanovich, about whom I am now writing a biography and whose virtuosity extended to his ability to summon the voice of Jimmy Stewart.