The ex-Emmerdale actress and her pro partner upset the odds and picked up a total of 30 from the judges after an impressive cha-cha-cha at the weekend.
Speaking on It Takes Two about Riley's opening routine, Horwood said: "I loved the fact that she has natural rhythm and she really actually dances.
"She has an inner motor. That's fascinating and we were not expecting it all."
However, he did offer some words of caution, adding: "I don't know how that will transfer in the ballroom. She'll be able to get her slow, quick, quicks right, but it's her posture and things like that she'll have to watch out for."
The judge also insisted that he enjoyed the first weekend of live shows, despite his "face and comments".
"I loved it. It was fantastic," he said.
