The woman, now a pensioner, said she was watching the ITV soap with her husband who remarked that Mr Roache, who plays Ken Barlow in the show, was "OK".
The witness said: "I said he wasn't.
"I said he's a dirty old man, is what I said."
The witness was the second alleged victim to give evidence against the 81-year-old TV veteran on day two of his trial at Preston Crown Court.
She told the jury that when disbelieving friends mentioned sex allegations circulating about Rolf Harris she told them: "You better believe it, William Roache would do the self same thing."
Mr Roache is accused of using his stardom to prey on young girls. He denies five historic counts of indecent assault and two historic counts of rape against five girls aged 16 and under. The offences are said to have occurred between 1965 and 1971.
The witness told the jury she was 16 when she was groped by Mr Roache in the women's toilets at Granada Studios in November 1965.
She said: "As I went in I suddenly was aware of somebody behind me and I turned round and he was right in my face, absolutely right in front of me. I could not move.
"He more or less pinned me against the back of the wall."
She confided in her husband about 20 years ago, jurors heard. She made her complaint to the police in May last year after she saw a television report on the rape allegations that the defendant faced.
She said: "I thought that if I told my experience it might lend some credence towards her allegations.
"I felt if he could do that to me, he could do that to her."
Earlier the court heard from Mr Roache's first alleged victim, who said he had written a letter to the then 14-year-old girl weeks after he indecently assaulted her and asked her to reply "when you start school again".
She claimed Mr Roache forced her to perform a sex act on him in the men's toilets at Granada Studios in Manchester in the summer of 1965. She said she had visited the studio with a friend to take part in a children's talent show when Mr Roache pulled her into the toilets.
Weeks later, she said, she received a letter and signed photograph from the actor. She said that after receiving the letter she returned to the studios later in the year, telling the court that afterwards Mr Roache told her to wait outside and he would pick her up in his car.
She said she thought that she had also been indecently assaulted in his car but she had "no actual memory" of the episode.
Questioned by Louise Blackwell, QC, defending Mr Roache, the witness told the court she could not remember many details about how she allegedly got to be alone in the men's toilets with the accused or any detail of a conversation they may have had.
She told the jury: "I think it's because it's 50 years ago and a lot of things are unclear about that day, but some things are clear."
Asked about the second alleged indecent assault, the witness said that she could not remember any additional details about Mr 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?"
The witness replied: "No, I don't agree. 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?"
The witness said: "It is true, I'm telling the truth.
Miss Blackwell added: "The lapses of memory are because you are making it up?"
The witness answered: "I'm not making it up. I would have no reason to."
Mr Roache denies any sexual activity ever took place with any of the alleged victims.
The trial was adjourned until this morning.