When Amanda Knox looked into an Italian documentary film-maker's camera and recited the "to be or not to be" speech from Hamlet, the select few who viewed the film heralded it as a stellar performance. It was the debut no one saw, however, because authorities blocked the documentary film about women in prison once they discovered Ms Knox was the star.

But when the 21-year-old from Seattle takes the stand next week, she will have the biggest audience of her life. The whole world will be watching. "She will respond to all the questions and tell 'her' truth," said Luciano Ghirga, the Perugia lawyer representing her. "She's ready."

Prosecutors allege that Ms Knox, her Italian ex-boyfriend, Raffaele Sollecito, and the Ivorian-born drifter Rudy Guede stabbed, strangled and suffocated Meredith Kercher, 21, from Coulsdon, Surrey, in a struggle that ensued after the British student resisted a sex game on the night of 1 November 2007. Ms Knox also faces theft and slander charges in the case.

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Guede was convicted in a fast-track trial in October and sentenced to 30 years. Defence attorneys maintain he acted alone; his appeal is scheduled for November. After five months and more than two dozen hearings, the closely watched trial is now reaching its dramatic apex – with heartbreaking testimony from Ms Kercher's mother, father and sister yesterday and Ms Knox taking the stand next.

The back-to-back Kercher-Knox testimony mirrors how public opinion is dividing into two distinct camps: the "innocentisti e colpevolisti" or "Innocents and the Guilties". On the one hand there are the vocal, media-savvy friends and supporters of Ms Knox, completely convinced of her innocence, who say the case is plagued by shoddy police work and a controversial prosecutor. Others say the evidence is damning and criticise blind support of the pretty, all-American girl when the true victim is Ms Kercher.

Yesterday, few in the courtroom were unmoved as Ms Kercher's father, mother and sister described the pain, anger and void in their lives. Her mother, Arline Kercher, her voice quivering, spoke of the terrible effect on the family of her daughter's murder: "Not just her death, but the nature of it, the brutality of it, the violence of it. The great sorrow it has caused. It is such a shock to send your child to school and they don't come back. We will never ever get over this." Meredith's father, John, testified to his daughter's physical strength and karate skills. "She could have put up quite a fight," he said.

But while yesterday was a sad and difficult day of testimony, it is harder to predict how it will go this week for Ms Knox, whose spontaneous and sometimes quirky personality has at times worked against her. Her first declaration before the court, for example, was in response to Ms Kercher's British friends' testimony that they found it strange Ms Knox kept condoms and a sex toy in a transparent beauty case.

"I would like to make a brief clarification about the object in the bathroom. The vibrator. It exists," she said. "But it was just a joke, a gift from a friend before I came to Italy. It is a tiny pink rabbit about 15 centimetres."

Shocked journalists expecting a standard, rehearsed statement rewound their digital recorders to make sure they had heard correctly and began Googling the vibrator in question as if it were the alleged murder weapon. But as the trial has plodded on, Ms Knox has seemed more confident and more serious each of the three times she has spoken out. After police and interpreters testified about her interrogation the night before her arrest, for example, she claimed their version was not true, and that she had been pressured and mistreated.

But now that the prosecution has closed its case (228 piece of evidence and more than 400 traces of genetic material collected during six crime scene inspections), Ms Knox has more explaining to do – about why she implicated one African immigrant resident of Perugia, when another was involved, why she wrote conflicting statements, what she did that night, and possible explanations for why her DNA, blood and footprints were identified at the scene of the crime. Last month, the Rome forensic police biologist Patrizia Stefanoni testified that five mixed DNA samples – blood or DNA that tested positive for both Ms Kercher and Ms Knox – were found in various rooms of the small villa the two women shared. Their mixed DNA (traces of genetic material, not blood) were found on the 20in kitchen knife that prosecutors allege was used to stab Ms Kercher.

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"On the handle there is the profile of Amanda Knox and on the blade there is the genetic profile of the victim," Dr Stefanoni told the jury. There were also footprints that Dr Stefanoni said were made in Ms Kercher's blood, where Ms Knox's genetic profile was also identified. Forensic police say a bloody footprint on a bathmat is compatible with Mr Sollecito's and his DNA is on a bra clasp found 40 days after it was initially noticed by investigators.

A second forensic geneticist testified for the Kercher family's civil case on Friday that she agreed with Dr Stefanoni's analysis and reconfirmed the prosecution's reconstruction of a sexually motivated attack by more than one aggressor.

But defence teams have hired their own experts, who will try to pick apart the evidence: Ms Knox's blood in the bathroom cannot be dated – perhaps she was menstruating. She regularly used the utensils in her boyfriend's home, so of course her DNA is on the knife handle. And, in cross examination of the prosecution's witnesses, Ms Knox and Mr Sollecito's attorneys revealed crime scene mistakes that could have led to inadvertent contamination of some evidence. Her lawyers intend to ask for independent evaluation of the most contested evidence.

Ms Knox, however, is likely to focus on taking back something she lost control of early in this case: her image. In the days immediately after her arrest, she was portrayed by the prosecutors and the press, particularly in Britain and Italy, as a twisted and diabolical sexual huntress. Her family responded by hiring a Seattle public relations firm, which waged an international campaign using private investigators, lawyers and TV personalities to argue that she was innocent and being railroaded. A Friends of Amanda group was founded to help "turn around the supertanker of character assassination and negative stereotypes".

Multiple witnesses have testified about what they deemed Ms Knox's suspicious behaviour – odd domestic habits, the cartwheel and splits she did while waiting in police headquarters, the overt canoodling with her boyfriend the day after her roommate's murder.

"It seemed like a person who had gone crazy," Ms Kercher's best friend, Amy Frost, told the court in February. But those who know her well say her personality as portrayed in court and by the press is at odds with the woman they know – an outdoorsy Seattle college girl who got good grades, liked yoga, soccer and rockclimbing, partied occasionally and kept extra condoms in her bag just in case. So what?

Out of context in a provincial, and very Catholic city such as Perugia? Yes, but is this wholesome, angel-faced pixie a cold-blooded killer? Some find it hard to believe. Especially in Seattle, a city that is twinned with Perugia. The two places are a natural match, both centres of learning and culture. Seattle is also affluent, with hi-tech heavy-hitters such as Microsoft, Nintendo, Amazon and Boeing among the major employers. Perugia is known for its chocolate; Seattle for its coffee (Starbucks was born there).

But Amanda's Seattle was neither affluent nor alternative. She played football while attending Seattle Prep, a private Jesuit high school, and got good grades, despite having to hold down two jobs in order to make ends meet. She enrolled at the University of Washington, where she would study German, Italian and creative writing. Again, she kept her grades up, while working in a university district coffee bar. Like most college students, she partied, too.

Ms Knox's mother and father (now separated, but still friendly) live a mile apart in middle-class homes in the Arbor Heights suburb. While comfortable, the families are not rich, and the financial strain of the past year has been heavy. The Knoxes have made multiple trips to Italy, paid lawyers' fees, retained some of Italy's best experts and hired a public relations company. Family members take turns attending the trial and share a car to get around Perugia. Interviewed earlier this year in Seattle, three of Ms Knox's closest college friends said that they had been saving up money to travel to Perugia this summer to testify as character witnesses. First to testify will be Andrew Seliber, who studied psychology and once lived in the same residence as Ms Knox.

For next week, however, her supporting cast will have to exit stage right as the spotlight turns on to Ms Knox herself. As a communal witness, she will be asked questions by all the parties. As the accused person on trial, however, she is not required to answer questions, or even pledge to tell the truth. Ms Knox can say whatever she pleases. She is expected to start her testimony in English, but is also fluent enough to speak in Italian, as she has in prior statements. Legal observers say that exposing Ms Knox and her genuine-but-unpredictable personality on the stand is risky – it could go either very well or very poorly, but it was obviously a risk her lawyers were willing to take.