Shetland star explains Tosh and new detective's "unusual" dynamic
"I'm not sure I can think of lots of examples of a relationship like that."
Her arrival forces Tosh to press pause on her own case and work closely with her, but the two women do not operate in the same way, which creates an interesting dynamic.
"They're both incredibly driven, they're both experienced, but they're coming from different backgrounds, they've had different journeys to get to where they are, this level of seniority," Alison O'Donnell told RadioTimes.com exclusively.
"I've described it as being a very restless relationship, it never really settles down. And I think it's quite an unusual dynamic for TV. I'm not sure I can think of lots of examples of a relationship like that. They don't immediately become best friends, but they're not enemies, they have a common goal.
"And they don't have time to get to know each other, they're straight in at the deep end. And sometimes they knock against each other, and sometimes they really hit a rhythm.
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"It just depends what they're faced with on that day, what point they're at in the case."
Exploring the relationship between Tosh and Ruth was a "really, really rewarding" experience for O'Donnell, who had spent the previous nine years acting alongside Douglas Henshall as Jimmy Perez.
"You could never get complacent with it," she said of the show's new chapter. "There was always something new coming up the path.
"I never got to the point where I was like, 'Oh actually, I really know what I'm doing with Ashley' because the next day we would be filming something completely different again."
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Ruth, who left Shetland many years ago, reluctantly heads back to the isles to retrieve a key witness, with the case pushing everyone to their limit.
"So many things get thrown up, and they both move from the front foot to the back foot," said O'Donnell, adding: "And from the absolute word go, there's so much jeopardy there. They're up against it in terms of time, in terms of stakes, so there isn't the luxury of settling in. They just have to be really, really light on their feet.
"Both of them to try to keep abreast of what's happening in the case, keep abreast of what each other is thinking and how that relationship is going, so it just never relents. It keeps them on their toes from the word go, right the way through to the end."
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