The four-part ITV miniseries, due to premiere later this month, follows the story of a murder and is based upon Cross's own book Burial.
That story is, in turn, inspired by a harrowing nightmare that Cross had, the actor detailing the dream in a new Q&A with Digital Spy and other media.
Host Boyd Hilton asked the Sister creator: "This is adapted from your own novel, Burial, and I read that the inspiration for that novel was you having a dream in which you murdered someone in a forest, which is kind of what happens, to some extent, in the early stages of this show. Was that true? You dreamt that you murdered someone and you were so unsure as to whether it had actually happened or not that that inspired the novel. Is that right?"
Cross then answered: "I want to say, even now, I hope I dreamt it! I was very young. I was 16 or 17 and I was out of my head on cider in Bristol.
"We used to drink cider in gallon containers because it was 32p a pint and I was making my way back to my sister's house and there was a shortcut... you can either walk round a very long road... or you can take a shortcut through the woods, through these concrete steps through the woods.
"And I just woke up the next morning, not with any sense I'd had a dream of any description. I woke up with a very, very clear memory of coming across a homeless man asleep on the steps who I randomly stabbed to death.
"And I woke up the next morning and, as I say, there was no sense of it being a dream, I just remembered doing it, to the extent that I lived in degrees of fear for certainly the next week."
The creator continued: "There was no blood on me, I have to say. I did carry a knife in those days... there was no blood on the knife, there was no blood on my clothes, there was no blood anywhere, but I was still looking out the window for the police, I was scanning the news. I was absolutely certain that I remembered randomly killing this man.
"I genuinely find this difficult to talk about... and when the internet was invented, one of the first things I did... was search the old newspapers, so that was the inspiration."
The Sister will premiere on Monday October 26 on ITV at 9pm.
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