"He was obviously just testing my Chinese." She passed the test. Charlton secured the role, but admits she found it difficult at first, her previous acting experience being predominantly theater based.
"It probably shouldn't have come as such a surprise, but it was very hard work, getting up at 4 am to get hair and makeup done, having to do an hour, sometimes two hours more (of hair and makeup) than the guys," she says.
Her Chinese co-stars say she took the long hours in her stride.
"When I was filming with Isabella, I could feel that she is very dedicated, especially since Chinese is not her native language", says Cao Yunjin, a comedian and actor. "She is always eager to learn and always open to advice and new ideas. I believe she has a great future."
Charlton grew up in Hong Kong, where her British parents were then living, but later moved to the UK to study law. She became involved in theater at university, and the experience convinced her that her heart was in acting - and in Asia, so she packed up and moved to Beijing.
She arrived in the Chinese capital in 2012, securing small roles in two historical TV drama series, Eastern Battlefield and Haitang Yijiu. She also started to develop an interest in xiangsheng, or cross-talk, a kind of Chinese standup comedy in which two people engage in onstage banter back-and-forth using rhyming slang and telling stories for laughs.
"I had done standup comedy before in English, so to do it in Chinese seemed like a natural progression," Charlton says.
"Beijing's a great place to get into xiangsheng, there are great teachers, and then comedy brought me more into acting."
Charlton is currently filming a movie called The Cairo Declaration, about China's role in World War II. She is playing Mary Churchill, the youngest daughter of late British leader Winston Churchill. After that, she has plans to tour Taiwan with a lead role in a Chinese stage production of Shakespeare's Macbeth.