Slow Horses Season 6 Episode 5 proves that Apple TV's spy thriller series has never been short on sharp dialogue, unexpected character dynamics, or moments that turn an otherwise ordinary exchange into something worth talking about. The recently released penultimate episode saw River (Jack Lowden) and Sid (Olivia Cooke) get into heated and flirty banter, which eventually led to a much-awaited kiss.After Diana's deal with the GRU, Sid learned she was to board a plane to Bulgaria that would likely take her to her demise. The urgency of the situation saw the duo attempt a great escape from the MI5 Agents, until River stayed behind and sacrificed himself for Sid, and the couple parted with a kiss.What makes River and Sid's exchange particularly intriguing in Slow Horses Season 6 Episode 5 is the story behind how the scene came together. In an interview with TV Insider, head writer Gaby Chiappe revealed how Lowden and Cooke improvised River and Sid's kiss, making the moment even more special.An improvised scene stole the show in Slow Horses Season 6 Episode 5 View this post on Instagram Instagram PostSid and River's slow-burn romance came to a head in Slow Horses Season 6 Episode 5 when they found themselves in a military hangar with MI5 Agents following Diana's orders. While Lamb had figured out the sequence of events that led to Diana's deal with Vasilly, River was still confused. The duo wondered why Diana did not use the identity of a dead agent for her off-the-books Kazan operation. While Sid theorized that she was Diana's 'contingency plan,' River managed to connect her to the First Desk.Diana confessed to Sid in Slow Horses Season 6 Episode 5 that she had made a deal with the GRU to hand over the agent. While Diana advised Sid to take one for the team and said she would be home in a few months, Sid was convinced that the plane was taking her to her death, which eventually led to the scene-stealing moment of the episode.Fueled by the dire circumstances, Sid mustered the courage to confess to River that he was the only person she could trust. After a flirty exchange, carrying a lot of sexual tension and yearning, Sid was whisked away to board the plane. River had a realization that he could not let Sid slip away. While River and Sid managed to escape the Range Rovers, River knew they needed a distraction. He eventually sacrificed himself and asked Sid to run, but not before a parting kiss.Speaking to TV Insider, Gaby Chiappe revealed that River and Sid's scene-stealing kiss in Slow Horses Season 6 Episode 5 was improvised by the actors:“I love it. And then the actors improvised it. That wasn’t in the script. I think they thought they needed it, and I thought it was absolutely brilliant when they did it. The kiss came from them. It’s very lovely.”The kiss works because it was not only a payoff of a romance building since Season 1, but the stakes were too high for both characters. While their earlier conversation led to the unexpected kiss, their eventual separation gave it a bittersweet ending.How does Episode 5 set up the Slow Horses Season 6 finale?A still from Season 6 (Image via YouTube/Apple TV)While Lamb gained intel on Diana's Kazan operation in Slow Horses Season 6 Episode 5, Slough House is likely still in danger, especially since Jane mysteriously vanished from the crash site, suggesting she is still alive.While Chiappe was tight-lipped, she teased a storm coming for the Slough House agents in the finale:“It’s like a sort of gathering storm. Everything is coming to bear on where everybody is gathered in that one house and all h*ll is coming for them. And we know that Slow Horses, people die, so you don’t think Coe is necessarily going to be the last on that body count. And who comes away from that and how is very much worth watching for, I would say.”As for River and Sid, while the couple almost confessed their feelings for each other, it remains to be seen if/when they will be reunited in the upcoming entry.The Slow Horses Season 6 finale will be released on Apple TV on October 21.For the latest scoops on your favorite TV shows and movies, follow SoapCentral.