WHEN Rachel Leskovac took on the role of Nurse Kelly Yorke in Holby City this year, she had no idea what an emotional rollercoaster ride it would turn out to be. Not only would her character turn out to be 'the Keller's Killer', a serial killer stalking the hospital wards, but she would also have to deal with the real-life drama of her close friend and colleague Laura Sadler dying in a fall at Sadler's boyfriend's home in June.

''Obviously it was a complete shock for everyone,'' she says of the tragedy. ''I'd only known her for three months but in that time we grew quite close. In this profession, you get to know people really quickly and with us being a similar age, we just really hit it off.

''She was a lovely, lovely girl so obviously when she died it was a horrific tragedy. Working on Holby afterwards was strange because she'd been a part of the show for so long and to not have her there was really sad. She used to make everyone laugh and I loved doing scenes with her.

''But you have to carry on don't you? People that knew her said she wouldn't want us all moping around and feeling sad. She'd be like, 'Come on, get on with it'.''

Leskovac and the rest of the cast have been doing their best to keep Sadler's memory alive. As well as keeping a photo of the 22-year-old actress on set, some of the cast members, including Leskovac, recently took part in a fun run for the Laura Sadler Memorial Fund. The fund was set up by Sadler's mum, Sonja, and stepdad, Steven, to help underprivileged children get into acting.

''I did a three-mile run, or maybe I should call that a walk,'' smiles Leskovac. ''But other people, nutters I'd call them, did a 13-mile run. Why on earth would you want to do that?'' she laughs.

Holby City has been exhausting Leskovac, 26, in other ways. Although her beginnings in the hospital drama were inconspicuous - Kelly arrived in Holby with her younger brother, healthcare assistant Nic, and seemed the perfect nurse - things have now taken a dramatic turn, as Kelly has been revealed as the serial killer who has been discreetly murdering patients on Keller's Ward.

''When I got cast, I didn't know that I was going to be playing a killer,'' says Leskovac. ''I knew she came from an abused background, but that was it really. When I turned up on the first day, I was introduced to some of the cast and they were going 'Oh you're the killer!'. I got sent to the producer and he said he was really sorry I had to hear about it like that and told me what the story was going to be.

''I was being really professional and saying, 'That sounds really interesting,' but inside I was thinking, 'Yes! This is so cool!'.''

After failing to implicate Ward Sister Kath Shaughnessy, Kelly got panicky and confessed her dark secret to Nic, hoping he would run away with her before the police closed in.

The siblings are close - Kelly looked after Nic after their mother walked out and their alcoholic father died at their home - but Nic could not come to terms with what his sister had done and begged her to give herself up. She said she would but gave him the slip, instead going to their mother's flat to finally confront her about her abandonment.

''Kelly started killing because she has all this anger and hatred inside her for her mother,'' explains Leskovac. ''She just can't help it. She's not psychotic or schizophrenic - she's got psychological problems.''

Now Kelly's mother bears the brunt of the nurse's wrath. She arrives at the hospital with multiple stab wounds but survives, much to Kelly's chagrin, and she tries to kill her again. But in the nick of time, she's caught by Kath. Though Kath corners her in an operating theatre, Kelly once again escapes, and, knowing the game is finally up, heads towards her mother's flat to end her life.

''She smashes the place up big style then breaks down,'' says Leskovac. ''Then Nic comes to find her. She's in a right state looking at family photographs and releasing all these demons that have been in her all her life. She feels she has to end her life to give Nic his life back. In smashing up the flat, the cooker has come loose and gas has been circulating around the flat for some time. Nic eventually talks her into leaving with him, but just as they're leaving, the phone rings and he decides to go and get it. Without thinking, he turns the light on. Kelly is far enough away to miss the worst of the explosion, but Nic is in the house.''

The action-packed storyline meant plenty of stunt work for Leskovac, some of it enjoyable, some of it not.

''When I was smashing up the flat, the adrenalin was unbelievable,'' she enthuses. ''I frightened myself doing it. Afterwards, the crew started looking at me differently,'' she laughs.

''The explosion was obviously much more dangerous so we had stunt doubles. We had to lie there for about 40 minutes on cold, wet concrete, which was really glamorous.''

Leskovac, who will be leaving Holby City early next year, isn't sure how she'll follow up such a demanding and rewarding role. But first, she's heading home for Christmas to her family in Bradford for a well-deserved rest. There, she'll be joined by her long-term boyfriend, musician David Tench.

''I love Christmas,'' she says. ''Not necessarily the buying gifts and the commercial side of it. But what I love is being with your family and the goodwill side of it. I'll be crying at something on TV, I'm sure. It's all I've done for the past six months, cry and kill,'' she laughs.

l The explosion episode of Holby City is on tomorrow on BBC One