In a deposition, Cosby himself said he had consensual sex with Constand, and only obtained Quaaludes for women he wanted to have sex with. Constand tells The Philadelphia Inquirer, "I did what I thought was right".
The prosecution asked for 13 other Cosby accusers to testify, but Judge Steven O'Neill has permitted only one in addition to Constand.
Cosby, once known as "America's dad" for his role as Heathcliff Huxtable on the 1980s hit TV series "The Cosby Show", walked into court on the arm of Keshia Knight Pulliam, who played his youngest daughter, the pigtailed Rudy Huxtable.
Defense lawyer Brian McMonagle immediately attacked what he said were inconsistencies in Constand's story, disputed that Constand was incapacitated, and made the case that she and Cosby, who was married, had a romantic relationship.
QUOTE: "I felt embarrassed because I had a secret about the biggest celebrity in the world at the time and it was just me, just my word against his, and I was very afraid". And, she said, her truth was to be at court and to be supportive. Her emotional testimony was meant to persuade jurors that Cosby demonstrated a pattern of behavior when he allegedly attacked Constand in 2004.
She said that before this incident she had refused two other "embarrassing sexual advances" from Cosby.
USA comedian and actor Bill Cosby is to go on trial for sexual assault in Pennsylvania later in a case that is expected to last about two weeks.
Cosby has said he does not plan to testify.
Cosby is accused of giving a Temple University employee drugs and then sexually assaulting her. And he said Ms Constand initially told police that she and Cosby had never spoken after the encounter, when, in fact, phone records show the two spoke 72 times after mid-January - and two-thirds of the calls were initiated by Ms Constand.
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Prosecutors are telling jurors in Bill Cosby's sexual assault trial that the actor has already admitted sexually violating an unconscious woman more than a decade ago.
Cosby will face a jury of seven men and five women.
Ex-Playboy model Victoria Valentino, Florida nurse Therese Serignese and former actor Lili Bernard, all among Cosby's accusers, were spotted in the courtroom's overflow room.
In February 2005, then Montgomery County District Attorney Bruce L. Castor Jr. decided not to bring criminal charges against Cosby, citing "insufficient credible and admissible evidence".
But the pills left the woman, Andrea Constand, unable to resist the alleged sexual assault as she lay on a couch in his suburban Philadelphia home in 2004, Assistant District Attorney Kristen Feden told jurors.
Feden, the prosecutor, warned the jury not to fall into the trap of confusing celebrities with the characters they play.
She said the comedian and star of TV's The Cosby Show gave her wine and what he called "herbal" pills to relieve stress. They will be sequestered for the trial. The cameras that dominated Simpson's trial aren't allowed in Pennsylvania courtrooms, but scores of photographers will be lined up outside the courthouse.
In this combination of file photos, entertainer Bill Cosby pauses during an interview in Washington on November 6, 2014, and Andrea Constand poses for a photo in Toronto on August 1, 1987. Cosby also offered to pay for Constand's education, which Feden painted as a concession of guilt.