Olivia Cooke in Slow Horses
Olivia Cooke plays Sidonie “Sid” Baker, the intelligence officer whose conversational ease makes her an unusually good match for River Cartwright. Sid can challenge him without turning every exchange into an office feud. Cooke gives their teasing a competitive edge: Sid listens, recognises when River is trying to impress her and refuses to reward his self-importance. Her opening scenes establish a capable operative inside an organisation that encourages viewers to expect incompetence.
In the first episode, watch how Cooke alternates between amusement and attention. Sid's lightness never means she has stopped assessing the situation. That quality matters when she and River move beyond desk work in Work Drinks. The performance makes her seem comfortable with uncertainty, while River is still trying to prove that he deserves a better assignment. Cooke supplies a different rhythm from Gary Oldman's abrasive Lamb or Jack Lowden's restless River.
What Cooke has said about the role
Speaking to TheWrap in 2026, Cooke said the series attracted her because it departed from the polished style of conventional British spy thrillers. She described the production as an especially enjoyable working environment. Her comments fit the first series' humour: these people can be observant professionals and still make an argument petty, awkward or ridiculous. Cooke does not need to make Sid solemn to establish that she is good at her job.
Reveal Cooke's Series 6 scenes and interview
Cooke reappears as Sid at the end of Series 6, Episode 2, before the reunion takes centre stage in Resurrection. The return changes the meaning of her disappearance after being shot in Series 1. Cooke told TheWrap that working with Lowden again felt immediately familiar; their instinct to needle one another returned without having to rebuild the relationship from scratch.
She also contrasted Adam Randall's approach to the modern thriller with the more elaborate preparation required by House of the Dragon. Randall left room for discoveries during filming, even within large action sequences. The result is visible in the movement between pursuit and conversation: Sid and River still have to escape, but their remarks carry the irritation, familiarity and unfinished business of people who knew each other before the danger began.
Career highlights
Cooke's film work offers sharply different characters. In Me and Earl and the Dying Girl (2015), she plays Rachel, the classmate whom Greg is encouraged to befriend after her cancer diagnosis. The film won both Sundance's Grand Jury Prize and Audience Award. Those are awards for the film, rather than an individual acting prize for Cooke. Rachel's place in the story gives the friendship emotional consequences beyond Greg's early reluctance to engage with other people.
In Cory Finley's Thoroughbreds, first shown at Sundance in 2017 and released in 2018, Cooke plays Amanda opposite Anya Taylor-Joy's Lily. Their reunion takes place in affluent Connecticut, and the film turns social ease into something unsettling. Amanda's bluntness provides a useful contrast with Sid's banter. Both performances depend on timing, but the surrounding stories give a flat response or an unexpected question very different implications.
Sound of Metal (2019) casts her as Lou, a musician opposite Riz Ahmed. In a Vanity Fair interview about preparing for the film, Cooke discussed learning guitar and working with a loop pedal for the opening performance. The role asks her to establish a working musical partnership before the story forces it to change. It is a substantial departure from the mannered social manoeuvring of Becky Sharp in ITV's Vanity Fair (2018).
For HBO's House of the Dragon, which began in 2022, Cooke plays the adult Alicent Hightower. The character belongs to a dynastic court in which public obligations constrain private choices. That makes the role a useful companion to Sid: both women operate inside powerful institutions, but Alicent's ceremonial surroundings and family politics demand a different pace from a contemporary intelligence thriller. Cooke's casting as Becky Sharp also shows how successfully she can make ambition entertaining without making a character uncomplicated.
Where to watch next
Start with Vanity Fair for a leading television performance built around wit and social ambition. Choose Thoroughbreds for a compact, darker study of friendship and emotional distance. Sound of Metal gives her a contemporary role with music at its centre, while House of the Dragon offers a sustained performance within a much larger fantasy ensemble. These four choices show different uses of her comic timing, reserve and ability to hold an audience's attention during an uncomfortable conversation.
Questions and answers
Who plays Sid Baker in Slow Horses?
Sid’s performer is Olivia Cooke, introduced in the first series. Her character profile and the opening two episodes are the best starting points for her first appearance.
Does Olivia Cooke play Alicent Hightower?
Yes. Cooke plays the adult Alicent Hightower in HBO's House of the Dragon, which premiered in 2022.
Which film won Sundance awards with Olivia Cooke in the cast?
Me and Earl and the Dying Girl won Sundance's Grand Jury Prize and Audience Award in 2015. Cooke plays Rachel.