The 28-year-old - who appeared as Myra McQueen's unhinged secret son Niall Rafferty in the Channel 4 teen soap - takes on the role of Davey Blake, an old friend of one of the medical show's new trainee doctors Lenny Lyons (Steven Miller).
Speaking exclusively to DS, Sloane said: "Davey's an old friend of Lenny's - they were best friends who lived in a children's home together when they were younger. They hated the place and made a vow to burn it down.
"It's been 14 years to the day since they made the pact and Davey turns up looking for his mate. Davey's got the mental age of a 15-year-old and has nervous twitches, so it's a really exciting and challenging part to be playing."
He joked: "I've got a beard again in this, which seems to be an ongoing thing! Every since the late nights, I've had roles with some sort of facial hair. People must have liked it!"
A Casualty source added: "It's going to be a huge episode and there's some kind of stunt involved."
Sloane begins filming at Casualty's home in Bristol at the beginning of next week for transmission in early January.
Earlier this year, the BBC confirmed that Casualty is to relocate its production base to Cardiff.