I had the opportunity to do a few indoor workouts with Marcus O'Sullivan while he was the reigning World Indoor 1500 Champ.
One thing that really stood out about Marcus (besides that he is one of the nicest people that I have ever met) is that he can change gears instantly. One second I was running a stride behind him at sub-4 pace and in the next second he was 5 yards a head of me. All I could think of is 'how many runners have seen him do this to them'?