Isla Fisher is writing a thriller with her mum.

The Great Gatsby star has admitted if she hadn't become an actress, she would have become a full time writer and after mastering the art of performing, she's now ready to try her luck at a screenplay.

The 37-year-old star has teamed up with her mother, Elspeth Reid - who helped her publish two teen novels, Bewitched and Seduced by Fame when she was 18 years old - to create a film, which she claims has been challenging, but amusing.

"My mum and I have actually been writing a script for a thriller together. It's really fun. She doesn't understand the rules of screenwriting, so I'm often like, 'What?! Mum, you can't kill off the protagonist on page 10!' She breaks all the rules, which makes it original at least. We've had a scream doing it," she said.