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.
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.