The stressed staff of Holby City Hospital are finally back this Christmas – but not as fans will expect as Casualty takes on a completely new format. The standalone special is all about blood, introducing several patients who urgently need transfusions – and sharing the real-life stories that inspired them. For star Elinor Lawless, who plays Stevie Nash on the BBC drama, the episode’s theme hit close to home. “I had a congenital heart condition when I was a kid,” she says. “I had open heart surgery when I was seven and because of that, I can’t give blood and it bothers me.

“It’s something so beautifully straightforward that we can do for each other and in that situation I was relying on other people to do the thing that I can’t do. I’m here because somebody decided to go to a blood bank that day – that’s just mind boggling.”

Now a mum herself Elinor, 42, finds the thought of her son Joe, eight, one day needing blood, terrifying. “I have a little boy and the idea of losing a loved-one simply because there’s not enough of the thing that would make an instant difference – that’s stuff of horror,” she says. “I’m grateful that people are doing this up-and-down the country and this episode is a vehicle to help give a message gently, but also to say thank you without shoving it down people’s throats.”

Set on Christmas Day, Casualty’s festive episode sees Siobhan, Dylan, Iain and the rest of the staff tackle a number of emergencies which desperately require blood – from a traffic collision that results in a road-side transfusion to a little girl awaiting a cancer diagnosis. Siobhan (Melanie Hill), Cameron (Barney Walsh) and Stevie find themselves treating a patient who refuses a transfusion over worries about infected blood – based on the real story of Andrew Evans, who caught HIV from a transfusion in the 1980s aged five.

“It was very emotional to film,” Barney says. “The patient we had in the bed didn’t want blood and it was running dry. To see the dramatization intercut with case studies of how this affects real life and see first-hand the impact that giving blood does make, it is a real honour to be a part of that.” Melanie adds: “It was really horrible when he was refusing the blood and the supply was going down – it was just that jeopardy. It’s Christmas Day and the emotions are heightened but as far as Siobhan is concerned, it is another day and we have to get on with it and try not to panic because this happens quite regularly.”

After hearing the real-life story of Bea, a little girl who relied on blood transfusions to stay alive whilst awaiting a heart transplant and features in the episode, Iain actor Michael Stevenson reveals he’s due to give blood himself. “I am booked in to give it this week after watching the show,” he says. “There’s a quote from her mum Cheryl where she talks about how Bea turned grey and after receiving blood, the colour came gushing back to her. Transfusions save people who have been in catastrophic accidents, but they’re also relied upon to keep people alive so they can be with their families for longer. It’s incredible.”

Working at the hospital during the festive season is a blessing for some of the staff – who want to be as far away from home as possible. “Stevie is burying herself in work because she’s terrified that if she looks outside of that, she might realise she’s completely on her own,” Elinor says.

“Siobhan loves being in the Emergency Department – she’s devoted her entire life to it and hasn’t paid enough attention to her husband with everything that’s happened there,” adds Melanie, referencing the end of Siobhan’s marriage to Rich Walker (Michael Keogh). “That’s where she gets comfort because every patient she helps makes her feel a little better in herself. She would probably dread going home.”

However, Iain’s Christmas reunion with his family is thrown into doubt when the paramedic’s car overturns on his way to pick up blood packs for the hospital. “His shift has finished and he’s going onto his family but a scenario crops up and out of the goodness of his heart, he tries to help,” Michael teases. “I first thought, ‘That’s yet another ambulance that Iain has managed to write off!’”

Filmed on an uncompleted stretch of motorway in Bridgend at six in the morning, the scene sees Iain’s fate hang in the balance – however, the fear of being killed off is a regular occurrence for Michael. “I’m always worried they’re going to do that. When I open the script, I look for the page where I find out whether I’m safe or not,” he laughs. “Being involved in all the stunts is a privilege and it never gets old. There’s something special about doing a night shoot, creating this little secret that nobody knows about.”

Despite being off air for three months, fans can expect more Casualty drama in the near future. “We’ve shot multiple episodes since so we know where we’re at,” reveals Charles Venn, who plays Jacob. “It’s very tasty stuff that we know the audience will like for sure. To be given a Christmas slot is a great way to come back into the show. It was a minor setback being off for three and a half months but we’re about to have a major comeback.”