Episode 8 of the long-running saga stunned Mark Hamill when he first read it, with Daisy Ridley (Rey) also revealing her own surprise at the storyline.
Now, speaking at a fan event, Johnson has provided a little more context to the upcoming twists and turns.
“It was very gratifying for me that Mark and Daisy and the whole of the rest of the cast when they read it were shocked by the things that happen,” he said.
“Because they’re kind of the first audience we have for the story and they know the characters better than anybody.”
He added: ”There are things that will shock the audience but you don’t want an empty surprise for the sake of it being a surprise.
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“You want it to be an emotional story that makes sense that surprising things happen naturally in.
“So I hope those are the kind of things that will make audiences gasp when they see this movie.”
Johnson has also been tipped to helm Episode 9, after the high-profile departure of Colin Trevorrow from the project.
The fraught relationships LucasFilm have had with several other recent directors within the franchise have made Johnson a real contender, given his seemingly drama-free rapport with boss Kathleen Kennedy.
A trailer for the movie is still being eagerly awaited by fans, following a teaser way back in April.
Star Wars: The Last Jedi is out on December 14 in the UK.