The Beach Boys member developed bizarre beliefs - such as producer Phil Spector trying to murder him - and became so paranoid he held business meetings in his swimming while under The Influence of drugs in the late 60s, but guitarist Al Jardine says his work as a songwriter never waned.
He said: "I was very concerned. I admonished him to stop taking drugs, but he'd already gone beyond that point. He had developed a lifestyle that was more detached from the rest of us and he'd started experimenting musically under the influence of those things. But the output was amazing.
"We were recording music that was spilling out of Brian's head at an incredible rate of speed."
Sessions for the album, 'Smile', were eventually scrapped after the rest of The Beach Boys felt Brian was heading into too psychedelic territory, and the work remained incomplete until he re-recorded the album and released it in 2004.
Brian says he has no regrets about the huge delay in bringing the album out, as it was too advanced for listeners back then.
He said: "We weren't upset at scrapping 'Smile'. We knew it was the right move. I think nobody would've liked it. People are much better at listening to music these days than they were in the mid-60s."
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