Just because Jack could give a great performance after smoking 47 joints, doesn’t mean the 47 joints are what made the performance great.
Bruce Campbell’s story on Conan is about how Jack Nicholson was allegedly very high while filming Easy Rider and so, Campbell apparently thought that if he was equally high, he could put together an equally good performance.
Alas, it seems that only Jack Nicholson is capable of stellar acting while stoned out of his mind.
It appears Mr. Campbell fell victim to the classic post hoc fallacy.