Shit the day that happens the world will stop spinnin' and Michael J. Fox'll come to a standstill
Michael J Fox can’t come to a standstill because he has Parkinson’s disease, which makes him perpetually shakey.
He means that if there was someone who could fill his shoes or take his spot then the world will stop spinning (impossible) and Michael J. Fox — who has Parkinson’s — will stand still (impossible). So, it is impossible to take his spot.
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