During the Arnold documentary available on Netflix, the actor explained why that joke didn’t make sense to him, contrary to what James Cameron thought.

The joke”I’ll be back” pronounced by Arnold Schwarzenegger in Terminator is probably his most famous, given that it was then repeated by the same actor in other contexts and films, but initially he did not agree on pronouncing it, contrary to James Cameron.
“Halfway through filming, we’re shooting the police station scene. The joke is: ‘I’l be back’. It wasn’t supposed to be an important moment at allrecalled Cameron, who co-wrote The Terminator with Gale Anne Hurd.It literally had to be a “No problem, I’ll be back“. For some reason, Arnold didn’t say ‘I’ll come back”. So I asked him, ‘Well, just say ‘I’ll be back’. Let’s make it simpler“.
Schwarzenegger then admitted in the documentary that he thought that “I’ll be back” sounded ‘funny’ because he was playing a cyborg and he thought that”I will be back“played more”is also a robot” and appropriate.
When Schwarzenegger made this suggestion, Cameron snapped at him. “He said, ‘Are you the writer? And I told him, ‘No,’ and he said, ‘Well, don’t tell me how the fuck to write’“.
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Cameron won the fight and beat Schwarzenegger “I’ll be back“, in Italian translated with “I wait outsidein the dub, it became the defining moment of the Terminator franchise and one of the most famous cinematic lines of dialogue in history.