Blue Lights star Siân Brooke teases "dark truths" behind unexpected season 4 return

In the spring of 2011, Siân Brooke played a single woman fed up with her nine-to-five job as a petrol pump attendant in a revival of the Mike Leigh play Ecstasy. She is routinely abused by the men who visit her in her bedsit – until she meets up with an old friend, played by Craig Parkinson, and they spend the night talking and crying. An optimist would hope that the two get together and live happily ever after.

Which, in hindsight, would be the strangest coupling: Brooke is famous for playing Grace, the social worker turned socially conscious cop in Blue Lights, while Parkinson is infamous for his portrayal of the crooked Matthew “Dot” Cottan in Line of Duty.

“Oh my God!” says Brooke, laughing at the idea of Grace and Dot co-habiting. “I can’t believe you saw Ecstasy! I had the most incredible experience working with Mike. I’d wanted to work with him for years. When I left Rada in 2002, lots of my mates – like Danny Mays – were working with him.”

Before her breakout role in the BBC series Sherlock, in which she played Eurus Holmes, Sherlock and Mycroft’s psychopathic sister, Brooke had established herself as a theatre actor. She played Juliet to Matthew Rhys’s Romeo for the Royal Shakespeare Company in 2004, and in 2015 – ahead of the final series of Sherlock in 2017 – she was Ophelia to Benedict Cumberbatch’s Hamlet.

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Along the way, Brooke learned how to deal with filming intense scenes. When Eurus is locked up in a glass cube in a maximum-security prison, Sherlock visits her and they both play the violin. “Benedict and I knew each other really well because we’d done Hamlet. You’ve got to have a laugh between those kinds of scenes or you’d go a bit crazy. Plus, Benedict and I weren’t exactly virtuoso violinists, so it was hard not to laugh at times.”

By the time she was cast in the first series of the Belfast police drama Blue Lights in 2023, Brooke had more than enough experience to bring both gravitas and humour to a challenging role. The cop show quickly established itself as an action-packed thriller that refused to look away from Belfast’s social and political divisions. The three rookie cops – Grace, Katherine Devlin’s Annie and Nathan Braniff’s Tommy – have to navigate a city in which the legacy of sectarian violence casts a long shadow.

Its realism induces the same kind of anxiety as The Pitt or The Bear, which is, Brooke says, what showrunners Declan Lawn and Adam Patterson were aiming for. “We wanted it to be gritty, to encourage viewers to walk in other people’s shoes. I fell in love with Belfast in the first season; we spend over four months there shooting each season and I miss it when I’m not there. I can’t deny the fact that, as an English woman, I am and will always remain an outsider, but equally I now call Belfast a home from home.”

In the fourth series, Grace, Annie and Tommy have to deal with a situation that is almost ungovernable, as anti-immigration groups try to shut down a migrant resource centre and Loyalists and Republicans clash. Katie (Tabitha Smyth), a young woman who is part of the anti-immigration protesters, live-streams all the action on her phone and posts about Grace on social media, thus compromising her safety and authority.

Can Brooke relate? She laughs. “I can certainly empathise with the social media storyline. Over the years, I’ve become more aware of people taking photos of me and I wouldn’t say it was a part of the job that I craved! This” – she waves her phone around – “is the catalyst for so much unrest in society. Everyone has the ability, at every given moment, to film and edit something as they choose.

"It’s something I fear for all of us, more as a human being than as an actor. It’s so tough for Grace to be violated in that way because she’s taken great care in presenting herself to the world as a wholesome, moralistic woman. I’ve always had a healthy dose of self-criticism about my work, but not to the extent that I’ve considered quitting. I use it to push myself, whereas Grace, who is usually resilient, is wondering for the first time if being a cop is for her.”

Brooke, 46, grew up in Lichfield, Staffordshire with a father who was a police officer and a mother who worked as a teacher. When first cast in Blue Lights, she talked to her dad as part of her research.

“My dad and I talked extensively about police procedural issues. I’ve been privy to the inner workings of the force my whole adult life, which has given me a greater understanding of Grace’s job.” Is it odd that he was an actual cop while she’s being paid to play one? She nods, laughing. “The cops are the real heroes, I’m just pretending! I never wanted to be one, but I do watch a lot of police documentaries. I love 24 Hours in Police Custody – I’m fascinated by how people behave under pressure.”

One of the key storylines in series four is the murder trial of Gerry Cliff (Richard Dormer), who was unexpectedly killed in the first series. Surprisingly, Dormer himself is returning. Is he somehow still alive or does he just reappear in flashback? “I can’t tell you! When we had the read-through prior to shooting, it was a real treat to have Richard back. I can tell you that with the arrival of Gerry, a lot of deep dark truths are uncovered.”

What we do know is that Brooke’s two children with husband Bill Buckhurst, a theatre, film and TV director, are too young to watch Blue Lights. At 13 and 11, the boys can’t watch much of her recent work, whether it be Trying, House of the Dragon or The Undeclared War. “It would be nice to do a role that made me cool in their eyes. At least my eldest is looking forward to the new season of Supacell, which I was filming around the same time as Blue Lights.”

Straight after she finished work on those two shows, Brooke went to Bristol to join the new Chris Lang drama Mavis Eccleston, a true story about a couple who make a joint suicide pact when the husband is diagnosed with terminal cancer. The husband, played by Jonathan Pryce, dies, but the wife, played by Penelope Wilton, survives and is arrested for murder.

It was a dream job for Brooke. “Chris is everything I hoped he would be: exceptionally intelligent as well as a great writer. The story happened 20 minutes from where I grew up and I’ll always champion stories that are from the Midlands. Assisted dying is very current and is something we should talk about, whatever your beliefs. Goodness me, it’s always better to talk.” Brooke plays the couple’s daughter, who is very much against her parents’ plans at the beginning. “She has a long journey, which of course Chris tells beautifully.”

After such a productive year, are there any particular roles Brooke would like to tackle in the next 12 months? “Oh, yes!” she says, enthusiastically. “I would love to be in a period series. I had Pride and Prejudice on a VHS tape as a teenager, and I watched it repeatedly. I was obsessed. I’ve done Shakespeare and Restoration comedy and now I’m ready for a bonnet and a carriage!”

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Blue Lights season 4 will air Tuesday 6 October on BBC One and iPlayer.

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