Really a movie not a theater actor, but Gene Hackman.
I was a member of the Theater School at De Paul University when he got the Oscar nomination for Mississippi Burning, and that very morning they rushed him and Joanna Cassidy (who was making a movie with him--I'm guessing in Chicago) to the school and hastily assembled the entire conservatory student body for a Q&A with him. He was very entertaining, of course, but also wouldn't talk about "the biz" and very, very dismissive of work like Superman and The Poseidon Adventure, and clearly uncomfortable with the fact that the movies we all remembered him from were. When someone asked "Aren't they even fun?" he made a snide joke about working with a blue screen. He wanted to talk Bonnie and Clyde and The Conversation. Now some of us surely had seen Bonnie and Clyde, which was rerun on television a lot, but there was barely a chance any of us--mostly 18-21 year olds, and young children when it came out--had yet encountered The Conversation.
I found him pretentious and dislikable, and it colored my opinion of his work for some time. But over the years, not only have I learned about his own bouts with depression, but also I realize we were as much the wrong group for him as he was the wrong speaker for us. Despite being a hugely accomplished actor, he wasn't there to give a class or to teach us about craft, and as I said, he wouldn't discuss the business side of the biz, so he really didn't have anything to give us. And we were too young to really know and appreciate the most important and challenging performances he had given. So my feelings have softened.
The person who really had something to offer us was Johanna Cassidy. DePaul instilled so much crap in us that if you didn't go to a prestigious school, you had no hope of making it in the business, that many students were shocked a woman who admitted she had studied so little had gotten the breaks she had. Someone asked how that was possible, and she fairly (and rightly) indignantly said "I got my headshot and my agent and I pounded the pavement!" We learned something from her that day. We really didn't learn anything from Hackman. Which is not to say he has nothing to teach.