Shortly before principle photography began the character of John was written to be a young man in his mid-twenties, with Kate as his sister. During a problematic casting process this idea was scrapped in favour of a father/daughter relationship after a shot-in-the-dark read through by the older, maturer, actor Paul Kelly.
The unique knock on the door by Trixie (long-short-long-short) is an homage to the film Brick - which shares a number of common themes.