The intense movie star, who plays the late country legend in the film, wanted to look like an amphetamine addict coming to terms with his drug problems and so went to extremes to make sure he was as authentic as possible.
But, by abstaining from sipping water, Phoenix blacked out on the set and ran into a prop, almost knocking himself out.
He says, "It was my attempt to get into the state of what that kind of detox must be like.
"He (Cash) was hallucinating, he thought spiders were on him, he clawed his skin. He really kind of lost it."
But, though Phoenix will own up to the measures he took to be authentic in the film, he's angry at some callous critics, who suggest he conjured up feelings he had when his brother River Phoenix died a decade ago for scenes in which he plays Cash grieving over the death of his sibling.
The actor fumes, "That's horses**t... John talked and wrote quite openly about his experience, so there was never a time I thought about my experience and whether I could identify."