Kate Middleton 'overstayed' on Downton visit

The Duchess with actor Jim Carter (Carson). Photo / Getty Images

The cast of Downton Abbey have spoken about a visit from the Duchess of Cambridge to their set earlier this year, revealing they chatted about chocolate and were "star struck".

Kate Middleton was heavily pregnant with Princess Charlotte and "overstayed" when she met with cast members of the popular British period drama.

The Duchess of Cambridge chats to actress Michelle Dockery (Lady Mary Crawley) and Joanne Froggatt (Anna Bates). Photo / Getty Images

"We weren't sure what to expect really," Laura Carmichael, who plays the character Lady Edith, E News at a pre-Emmys Tea Party in Los Angeles.

"We all got quite shy and she was sort of leading the conversation. It was lovely," Carmichael said.

"She's so normal. I know everyone says that but she really does try to make everyone feel at ease."

Carmichael said the cast lost their minds when the stunning Royal arrived.

"We started talking to her about the catering-I'm sure things she found really boring but we all sort of lost it because we were panicked and star struck. It was one of those weird ones."

Other cast members chatted to the Duchess about chocolate, and the expectant mum revealed she was a "big fan" of the show.

Kate chats to actresses Sopie McShera and Lesley Nicol. Photo / Getty Images

"I don't think she'd been on a film set before or a TV set so she was intrigued by how different it looked on the screen and in reality," said Jim Carter, who plays butler Carson.

"She's a big fan of the show... She overstayed. She stayed long. She was having such a good time."

Downton Abbey creator Julian Fellowes said her visit was akin to a "seal of approval."

"The Duchess is very charming and she follows the show so she was able to talk properly with all the cast and the crew," Fellowes told the US entertainment network.

"It was a great thrill and treat. We were very honoured."

While everyone at Downton seemed enamoured with Her Royal Highness, Fellowes insisted "she didn't get any spoilers".

The final season of Downton Abbey premieres tonight in the UK.