I don't have an exact quote but I once heard Rod Stewart say in an interview that he didn't think Robert Plant was a singer who rehearsed much before laying down vocals in the studio. In an other interview, Rod Stewart said that he thought that Led Zeppelin had a commercial edge that the Jeff Beck Group lacked in part because Page and Plant often used to come to the clubs to watch Jeff Beck's group perform, and took down mental notes of how the audience reacted.