A former assistant to Cosby's Hollywood agent was the first witness, testifying that the actor drugged and sexually assaulted her in a similar manner at the Bel Air Hotel in 1996.
Montgomery County Assistant District Attorney Kristen Feden started by going over the now well-known details of what Constand said happened in 2004. That's what this case is about. The allegations, all of which Cosby has denied, have essentially ended his long career.
"It's not a distraction, it can destroy a man", said one of his lawyers, Brian McMonagle.
The trial involves just one of those complaints, that of the former Temple University basketball staffer.
But because of Cosby's status in the showbiz world, Johnson kept silent. As a photographer, she generally avoids voicing her personal opinions about the subjects she covers, she said.
She planned to "put the pill under my tongue, pretend to swallow it, then go to the restroom and spit it out". "She was never incapacitated".
Constand moved to Canada and tried to get her life back to normal, Feden said, but she couldn't.
The first day of the Bill Cosby sex assault trial quickly spiralled into a bitter he said-she said courtroom brawl.
Prosecutors called Johnson to try to show Cosby had a prior history of drugging and molesting women. Three rows behind Allred sat Joe Cammarata, a DC-based attorney who represents seven accusers in a separate defamation suit against Cosby.
When she "came to", Johnson said that she was lying on the bed. "I came to support because this is where you hear the facts". She will be taking the stand sometime this week.
Her next memory was the assault in the bedroom, she said.
On a separate occasion she described how Cosby invited her to a hotel he was staying in. Can you tell us what she testified to today?
"He was offering me the pill to take and I said, 'Oh no". She had nightmares. Her parents realized something was wrong. "We were afraid. My father felt so powerless".
Bill Cosby's sexual assault trial continues
Throughout the rest of direct questioning, Johnson's voice trembled and broke.
After the incident, Kacey said, she listened to a phone conversation between her boss and Cosby in which Cosby complained about her work and encouraged Cosby to fire her. She traveled with Illius and her family to see Cosby perform a few times.
"I want to be the person that I would like to have if the tables were turned", Pulliam told reporters.
"This is an important case, a weighty case, and there's a lot of evidence", he said.
Prosecutors will also call a second accuser, whose name is not yet public, to bolster Constand's testimony. They said Cosby testified in the lawsuit only after being promised he could never be charged.
The defense used its opening statement to lecture the jury on the high burden of proof in a criminal case and warned them not to get distracted by his fame or by "finger-pointing" from the prosecution. He chatted with actress Keshia Knight Pulliam, who played his daughter Rudy on the top-rated "Cosby Show", as they walked together.
Johnson's emotional testimony, which came on the trial's first day in Norristown, Pennsylvania, is meant to convince jurors that the alleged attack on Constand in 2004 at Cosby's suburban Philadelphia was part of a pattern. Sometimes, they spoke for up to 40 minutes, he said.
"See a pattern?" McMonagle said.
Rashad and Cosby were so beloved as a TV couple, they teamed up again as husband and wife in the CBS series "Cosby", which ran from 1996 to 2000.
"I'm not going to predict what the outcome is", Allred said.
"She couldn't say no", Feden said.
Mr Cosby says his encounter with Ms Constand was consensual. According to Bill Cosby, however, any sexual behavior that took place in January 2004 was completely consensual.
Within minutes, she claimed to have "blurred vision" and no strength in her legs.
The I Spy star has admitted he drugged Constand and touched her genitals at his suburban Philadelphia mansion. She told police she did not consent to the touching.
Cosby starred in "Fat Albert and the Cosby Kids" and "The Cosby Show". In later years, Cosby became a public moralizer, speaking out against what he saw as the failings of African-American community in raising children. Two jurors are black. The cameras that dominated Simpson's trial aren't allowed in Pennsylvania courtrooms, but scores of photographers will be lined up outside the courthouse.