The actor appears in this Saturday's episode of the BBC One medical drama, playing a dynamic new paramedic named Rossy.
Dyer told TV Times: "They've asked me before but the roles were too obvious, like drug dealers. But when they contacted me about this, I jumped at the chance.
"I'm really honoured, because it's a massive show and I used to watch it as a kid."
The 34-year-old added that he would love to return to the show one day to portray a more permanent character.
"I love the idea of playing a surgeon," he explained. "It would be really exciting to be tested that much as an actor, to have that complicated dialogue and make the audience believe you know what you're saying.
"I did notice on Casualty that some actors who've been in it for years still haven't got a clue what they're talking about - that was a surprise!"
Dyer's episode of Casualty airs on Saturday at 9.10pm on BBC One.
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