Olivia Cooke Returns to 'Slow Horses' — and Wants a Showdown

Olivia Cooke returned to "Slow Horses" in the Season 6 finale after a four-season absence, and her character Sidonie Baker's reappearance — announced with two words, "Hello, River" — landed just before the Apple TV spy drama delivered a poisoning cliffhanger that leaves Jack Lowden's River Cartwright collapsed on a kitchen floor.

Sid's resurrection was not a total surprise. The character returns in author Mick Herron's novels, and Apple's promotion for Season 6, which concluded this week, made little secret of her storyline. What has changed is scale: "Slow Horses" has since accumulated a raft of Emmy nominations and a few wins, while Cooke's own profile has grown via her lead role in "House of the Dragon."

Season 6 puts Sid and River on the run from Russian sleeper agents over a single night. MI5 head Diana Taverner (Kristin Scott Thomas) once used Sid's identity as cover for an off-the-books assassination on Russian soil, and the Kremlin now wants revenge against all current and former Slow Horses — the nickname for employees of Slough House, the dumping ground for failed agents. The pair's flight runs from River's late grandfather's country house to Diana's beach retreat, which Jackson Lamb (Gary Oldman) appropriates as a safe house, through a barn, an arcade and an airstrip where Sid is nearly shipped to Russia. They share a long-awaited first kiss; minutes later, River collapses from apparent poisoning.

What did Cooke know, and when?

Cooke shot Season 1 in late 2020, one of the first productions back after lockdown. "Who would have thought it would have gone on for so long?" she told Variety, speaking from the U.K. She kept in touch with developments, receiving messages from Lowden — "You're coming back!" — without ever getting a clear date. Watching later seasons, she said, was "really fun, but tinged with jealousy."

She and Season 6 director Adam Randall built an off-screen backstory: Sid spent the intervening years in a covert MI5 rehabilitation facility, possibly in Cumbria — "sort of like the Vermont of England" — relearning how to walk and talk after the gunshot wound to the head that Season 1 viewers took for fatal. "It's been an incredibly lonely process that has, I think, cauterized a part of her," Cooke said.

Five weeks of night shoots

Her material was compressed into five weeks of night shoots in midwinter, right before the December break. "It just felt like we were making an independent film for five weeks," she said, praising Randall's run-and-gun improvisational style. Working mostly with Lowden, she called him "my favorite type of man, which is a very silly man" who "just chucks you a curveball, and then you try and grab it."

The injuries came from unexpected places. Stunts involving fences, gunfire and a moving truck surrounded by fire and gas canisters passed without incident; plain running did not. At 2 a.m., chased by a Land Rover while trying to match Lowden's stride, Cooke pulled both quad muscles and hobbled for two weeks. "When was the last time I got chased?" she asked. "Since I was like 12!"

What does the finale change?

Cooke reads Sid's loyalty to Taverner as "some real Stockholm syndrome" — Sid believes Diana saved her life — and her feelings for River as genuine: "River has kept her going, in a sense." She now wants a face-to-face confrontation with Kristin Scott Thomas's character that Season 6 withheld. "I want a showdown," she said. "It feels like that is a very cathartic thing that must happen."

With "House of the Dragon" entering its final season, Cooke says she wants theater and more work behind the camera: "I've been doing this for 14 years now... I want to find new avenues to go down because I'm getting a bit sick of myself."