Beowulf, The Tudors and Bedlam star Charlotte Salt, 26, has been cast as Sam Nicholls, a "fast-living, fearless and fiery" army medic who has a surprising link with another member of the ED team.
Meanwhile, 28-year-old actor Oliver Coleman - who has previously appeared in The Line of Beauty and The Other Boleyn Girl - will play paediatric specialist doctor Tom Kent. Billed as "gentle, calm and a brilliant children's doctor", Tom finds that his maturity at work doesn't always carry over into his private life.
Salt's first Casualty episode airs in mid-October, while Coleman makes his debut in January 2012.
Casualty's series producer Nikki Wilson commented: "Charlotte and Oliver are incredibly talented, much sought-after actors and we're delighted to welcome them to the new Casualty set in Cardiff.
"Both new characters are set to inject a shot of excitement into the emergency department and, as ever, both will have an air of intrigue about them."
Casualty continues on Saturday at 9.25pm on BBC One.
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