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The This Morning presenter was surprised with the opportunity to appear on the long-running murder mystery series earlier this year, after years of being a huge fan.
She filmed her scenes in May, but has confessed it wasn’t all smooth sailing.
Chatting to the show's star Neil Dudgeon on This Morning, Willoughby said being on set had been like a day at Disney for her.
Dudgeon said the cast and crew had been impressed with the TV star’s skills, but Willoughby chimed in to say that he was just being kind and that it was “not quite true”.
“I was so embarrassed to open my mouth and act because you know, I am not brilliant at that sort of stuff,” she said.
“So I walked around and to anybody that would listen I said my two lines repeatedly at them over and over!”
The presenter, 40, was offered a role on the long-running drama in March, when Dudgeon and co-star Nick Hendrix appeared on This Morning.
The pair went on the show planning to offer the star a role and, although Willoughby was none the wiser, her co-presenter Phillip Schofield was in on the secret.
When they spilled the beans Willoughby instantly said: "Yes!"
But she did joke that she would probably have to be killed off pretty quickly as she wasn’t much of an actress.
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