The Brookside actor, 52, is joining the ITV soap as the mother of Weatherfield mechanic Tyrone Dobbs.
Tyrone (Alan Halsall) had been told by his grandmother Evelyn Plummer (Dame Maureen Lipman) that her daughter and his biological mother Cassie is dead.
So when Cassie turns up in Weatherfield, Evelyn has to decide whether to come clean.
Speaking on Good Morning Britain, Sweeney said her character was "completely different" to her.
"I turn up on the cobbles, and I am a bit unruly," she teased. "I've got a few problems... major problems... what will happen?"
The actor said she grew up watching Corrie and that landing the role really hit her when she first walked on to the set.
"I have been a fan of Coronation Street since day dot," she said.
"I have been saying it for years. Whenever I get interviewed, they say, 'If you could do any soap what would it be?' and I have always said Coronation Street. So I have been manifesting it for about 15 years!"
Speaking when the casting was announced, Corrie producer Iain MacLeod said: “Claire bowled us over in her audition and the scenes with her, Tyrone and Evelyn will be electric.
"Her arrival will mark the start of a tumultuous time for Tyrone and the family and her impact will be felt across the street in the weeks and months that follow.”
Sweeney's first appearance on the soap will be on 28 June.
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