Marc is such a pro. He knows he's worked up and it could get weird, his first question is "do remember the first time you met one of your idols?". Perfect question to get them rolling and to remind Keith that he's been in Marcs spot before. Marc always gets a good interview.
Marc should understand that what makes Keith Richards is he is basically an artist. You shouldn't try and grasp too much from him otherwise it's stealing. It's based on simplicity and he just sticks with what works. Just think of the Chuck Berry DNA. That helps. Keith had an attitude as well, and confidence. He also has a large vocabulary from blues music, but again, keeps it simple. It's a feel.