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."
"I loved it. It was fantastic," he said.
Strictly continues this weekend (October 13) on BBC One.
Watch Lisa Riley's first performance on Strictly below: