My interaction with Witten has never been more than a "Hello" at a conference. But I heard a lot about him from others. Scientists talk about science and other scientists at lunch!Ed Witten is an extremely approachable person. He always replies to emails, even from strangers. I remember writing him about the graduate program at Princeton and he replied with some useful information. He is at IAS and probably doesn't participate in Princeton graduate admissions. A friend, who was visiting IAS, saw Witten working in his office. He didn't have any questions, but just wanted to say 'Hi' and maybe chat with Witten. Witten was very welcoming, he left his work and ended up chatting with him for more than 30 minutes. Another friend went to Witten's office and asked a question to Ed. When he started explaining, Witten realized that this guy doesn't know any mathematics. He didn't give up, he tried explained, asked some questions to get an idea of the background of the student and then made suggestions on what to read. As an aside, Ed Witten once corrected a friend on mine (who is Indian) on Indian history!