Rivals star Victoria Smurfit has said Dame Jilly Cooper was "our queen", while reflecting on the "awful day" she found out about her death during the filming of the show's second series.
The Disney+ series is based on the best-selling novel by Dame Jilly, who died unexpectedly aged 88 last October after suffering injuries from a fall.
Irish actress Smurfit, 52, plays Maud O'Hara - the wife of temperamental but brilliant journalist Declan - in the award-winning drama, which also stars David Tennant, Aidan Turner and Alex Hassell.
In an interview with You Magazine for The Daily Mail, she said the show's producer Eliza Mellor interrupted filming to announce Dame Jilly had died at her Gloucestershire home.
Smurfit said: "It was a really awful day. Jilly Cooper was our queen.
"It's a testament to how youthful she was that, when the cameras stopped, I had this awful feeling someone had died but it never crossed my mind it was her.
"Imagine Paddington Bear with a twinkle in the eye, that was her."
Dublin-born Smurfit went on to praise Dame Jilly's "genius" writing, and described the author as "like sunshine with feet" who "saw the best of everybody".
She also reflected on Dame Jilly's memorial service, which was held at Southwark Cathedral in central London in January, and said: "After the service these men appeared from the side with trays of champagne and I'm thinking: 'Wait, we're in a cathedral here guys.'
"But it was just so Jilly. You never felt such love before."
Set in the 1980s, with the backdrop of the Cotswolds countryside, Rivals follows the high-stakes world of British television as careers, marriages and reputations hang by a thread when professional and personal lives collide.
Dame Jilly was an active part of its production and served as an executive producer.
Shortly after her death, her close friend the Queen visited the set of Rivals in Bristol to pay tribute to Dame Jilly, and also met members of the cast and crew including Smurfit.
Recounting the visit, Smurfit told the magazine: "The Queen was due to come but then her best friend died so she could have cancelled, right? But she didn’t and she was delightful and hilarious.
"You could tell why she and Jilly were best mates. The Queen has also got a twinkle. You got the sense there’s a fun gal in there."