At the time of the film's production, Noel Redding was in the middle of a lawsuit against the Hendrix estate over what he felt were unpaid royalties, which is why he chose not to participate and only appears in archival concert footage.
Eric Clapton was in the first phases of his heroin addiction in the early 70s. Despite his smooth and articulate delivery, close-ups during his interview show his eyes are heavily dilated and constantly darting back-and-forth.