Crystal plays Dr Lily Chao, who has to care for Gary’s mysterious character Val Kildare during a scary day at the hospital, which sees two patients killed.
“Val is a strange man – a man that Lily suspects of wrongdoing as well, so it’s a frightening situation for her,” says Crystal.
“But from my point of view when I was filming it, I didn’t mind at all – I mean, it’s Gary Lucy! There was a scene that Amanda [Henderson, who plays Robyn Miller] and I had to film with Gary, and we both got a little bit giggly.”
Despite coming from Hong Kong, Crystal says she’s always been a Casualty fan: “I came over to the UK when I was 11 years old to go to boarding school, but it wasn’t until I was a bit older that I was allowed to watch television.
“I remember staying up to watch Casualty on a Saturday night. A lot of my friends around the world can get the BBC, so they’ve all been watching the series for years, too.
“I took a DVD of my first episode home to show my mum, but she doesn’t speak much English, so she didn’t really know what was going on. She just kept asking me why I was angry all the time.”
Casualty, BBC1, Saturday, November 15, 9.15pm.