Jim Carrey was so intense about his portrayal of Andy Kaufman during the filming of "Man on the Moon" -- that he actually spat in Jerry "The King" Lawler's face on set ... this according to the wrestling legend himself.
Lawler -- who famously "fought" with the real Andy Kaufman back in 1982 (see original smack talk video below) -- was in NY this morning and explained how Carrey's method acting technique led to a serious confrontation on set.
According to Lawler, Carrey desperately wanted "The King" to put him in a piledriver for a stunt in the movie ... but when "King" refused, Carrey (refusing to break character) flipped out and launched a loogie right in his face.
The spit-shot caused a firestorm on set, says Lawler, and the former wrestling champ explained that he was sure the outburst meant he'd be axed from the flick. Fortunately, he wasn't.
No word if Carrey ever apologized once he allowed himself to snap back to reality.