We all love Louisa and are so happy Carolyn Catz is the actress portraying her in “Doc Martin.” Here’s a story with photos in Showbiz CheatSheet about how she almost didn’t get the part because of her past roles.
“Martin Clunes, 58, spoke with The Los Angeles Times in 2015 about auditioning actor after actor until Caroline Catz read for the role of Doc Martin’s love interest. He knew right away the dark-haired Catz was the one.”
"Doc Martin" with Martin Clunes Fan 
What the article didn’t point out, I think, was that Martin Clunes felt a certain chemistry with Caroline Catz; that he could work with her on this series, which was to become a comedy-drama, even though her past parts didn’t qualify as such. Screen chemistry wins out on a successful paring. The viewer sees it.