The actress's new memoir, 'Rebel Rising,' is due out in a few days ... and People Mag obtained an excerpt -- which features descriptions of things Rebel claims happened on the set of their 2016 flick 'The Brothers Grimsby' ... including a lot of inappropriate behavior.

She writes, "It felt like every time I’d speak to SBC, he’d mention that he wanted me to go naked in a future scene. I was like, 'Ha, I don’t do nudity, Sacha.'"

RW goes on to describe one particular alleged incident on set, in which she claims he summoned her one day to film an extra scene in Cape Town -- but then alleges he dropped his pants in front of her and said, "Okay, now I want you to stick your finger up my ass."

Rebel says she was shocked, and figured a way to get out of it ... writing, "I was now scared. I wanted to get out of there, so I finally compromised: I slapped him on the ass and improvised a few lines as the character."

In response, People reports Sacha's team provided nine anonymous statements from fellow crew members that backed the notion the scene was in the script, and that Rebel knew exactly what was going on ... namely, her character was intentionally trying to do this.

Rebel, however, counters that ... writing in her book that she actually talked to a lawyer about it all -- and in the end, she says she bit the bullet and buried the whole thing by laughing it off. One thing she does say ... she told producers she wouldn't be promoting the film.

In the end, the film didn't perform very well ... and Rebel says she considers that karma. The reason she's speaking out now, she adds, is to encourage women to speak their truths.

Of course, this all comes on the heels of Rebel recently naming Sacha as the "a**hole" whom she dedicated an entire chapter to in her book. While the excerpt here seems to be the juiciest part of her claims, she's presumably going to lay out the entire 'Grimsby' saga.