For those who are even more curious, Carlo Verdone (who was a friend of Leone) made some documentaries called "Verdone racconta Leone" (see for example this yt video). These documentaries are packed with anecdotes (Verdone told lots of stories in other occasions too!) and fun stories.

One of my favorite is how Leone convinced Clint Eastwood to use the cigar, which is probably now an unforgettable part of the way we remember Eastwood's character.