She said the actor - whom she dubbed The Roache - groped her in a room at Granada Studios in Manchester after being led past the Rovers Return pub on the set.
In a video recording of her police interview played to jurors at Preston Crown Court yesterday, she said she thought she was about 14 when Mr Roache arranged studio passes for her and her younger sister.
She said: "He came over to me and stood really close to me and put both hands on my breasts.
"He took his right arm off and he had gone up my skirt and into my knickers.
"I started worrying. I knew I couldn't run."
The woman added that Mr Roache - who has played Ken Barlow since the launch of the ITV soap - also told her: "Perhaps next time."
She told the interviewing woman officer: "He was like an octopus. Hands everywhere."
Mr Roache, 81, of Wilmslow, Cheshire, denies two counts of rapes and five counts of indecent assault involving five complainants aged 16 and under on dates between 1965 and 1971.
The woman said she did not tell her sister - who also claims she was abused by Mr Roache on a separate occasion - about the alleged incident until a few weeks later.
She said when it was announced that Mr Roache had been arrested on suspicion of rape, her sister rang her and said: "Have you heard the good news?"
"She said are you going to the police and I said 'No'. I thought 'Who is going to believe me?"'
The officer asked: "What made you come forward?"
She replied: "It was (my sister). She said we have got to nail him, we can't let him get away with it.
"I thought if she is going to come forward then I will."The trial continues on Monday morning.