The complainant gave evidence at Preston Crown Court on Wednesday and told the jury Roache had forced her to perform a sex act on him in the gent's toilets of Granada Studios in Manchester in the summer of 1965.
She had visited the studio with a friend to take part in a children's talent show.
The witness told the court how she had met Roache, who plays Ken Barlow in the long-running soap, in a dressing room before he later pulled her into the nearby toilets.
Weeks later she said she received a letter and signed photograph from the actor.
The actor thanked the young girl for her letter and the "nice things" she had said in it.
He then added: "I am away for three weeks now but I would like a letter from you waiting for me when I get back.
"Write to me when you start school again and tell me more."
Prosecutor Anne Whyte asked her: "How did that make you feel?"
She replied: "I suppose I was flattered at the attention.
"I didn't understand some of it. I had not written a letter. I left a note as well as my address.
"The 'tell me more' didn't make sense. I don't know what it referred to."
She said that after receiving the letter she returned to the studios later in the year, telling the court that Roache told her to wait outside and he would pick her up in his car.
He later parked near a railway embankment in Stockport and she remembered Roache asking about her age and when she would turn 16.
She said she thought that she had also been indecently assaulted in his car but she had "no actual memory" of the episode.
Louise Blackwell QC, defending Roache, then cross-examined the witness, who admitted she could not remember many details about the corridors she and her friend wandered in at Granada Studios or how she allegedly got to be alone in the gents toilets with Roache.
She also said she could not remember any detail of a conversation they may have had before and following the first alleged assault.
The witness was then questioned about the second alleged indecent assault, where she had been invited to Granada by Roache, but could not remember how the arrangement was made, and taken on to the set of the Rovers Return on Coronation Street.
Roache is then said to have asked her leave the studios and arranged to pick her up in his car on the street outside.
The witness said she could not remember any additional details about Roache's car or the journey in it that day.
Miss Blackwell said: "Would you agree, you have some very alarming lapses in your memory about these events?"
"No, I don't agree," the witness replied. "I think it would be strange if I remembered every detail of 50 years ago. I remember the unusual things that stand out in my mind, the unique things."
Miss Blackwell continued: "The other alternative is it's just not true?"
"It is true, I'm telling the truth," the witness said.
Roache, 81, of Wilmslow, Cheshire, denies five historic counts of indecent assault and two historic counts of rape involving the complainants who were aged 16 and under.
The offences are said to have occurred between 1965 and 1971.
The trial continues.