Speaking to DS at the Cannes Film Festival, Dekker explained that he intends for his former on-screen mother to take on the lead role in the thriller.
"I've written a screenplay with her to play the lead role," he said. "It's actually called Sick and it's about a woman who's married with a daughter. Things consistently terrible keep happening to her and you slowly realise she's bringing it on herself. It's sort of a question of why. It's a pretty psychological thriller. It's a hard role but she's good enough to tackle it."
Dekker, who also directed Headey in the film Whore and co-starred with her in Laid To Rest, added that he had "the time of my life" working on the short-lived Terminator TV series.
"We were all mad on that show and tired constantly," he said. "Lena and I were two badly behaved children so that was what you had to watch out for, constantly getting into trouble and playing games."
> Click here to watch our full interview with Thomas Dekker