"Tomorrow I'll be cross-examined rather fiercely in the court. I'm nervous about that as I want to do well," she told Indonesian reporters at a press conference.
Asked what she thought of Indonesia's claims that her husband and four other Australia-based reporters were accidentally killed in crossfire rather than executed in cold blood, she said: "That's been rubbish for 35 years".
"They were just doing their job like you are."
"Balibo," the first feature film ever made in East Timor, premiered in Melbourne last July before an audience including East Timorese President Jose Ramos-Horta, who says Indonesian forces murdered the reporters.
Starring Anthony LaPaglia, it tells the story of the five journalists killed when Indonesian troops overran the East Timorese town of Balibo in October, 1975, and a sixth who died later in the full-scale assault on Dili.
Jakarta has always maintained that the so-called "Balibo Five" died in crossfire as Indonesian troops fought East Timorese Fretilin rebels.
Indonesian banned the film but groups including the Alliance of Independent Journalists (AJI) have launched a legal challenge against the censors' decision.
"A film should never be banned in a country which is a democracy. Any organisation that tried to ban what the people want to see is making a mockery of democracy," Shackleton said.
"This is about the film and the rights of the people here to watch, think, believe and say what they want, not what the government wants them to do.
"This film lets the cat out of the bag, you can't keep it quiet any longer, the cat escapes. They have made a problem if they want to censor the film. I trust the Indonesian people to make up their own mind."
Although 25 years have passed since the reporters' deaths, the incident remains a sensitive subject for the powerful Indonesian military, particularly officers from those days who now harbour political ambitions.
An Indonesian military spokesman has described the film as "very hurtful".
Australian police last year launched a war crimes probe into the deaths, prompting President Susilo Bambang Yudhoyono to warn that such an "inaccurate mindset" could damage Canberra's relations with Jakarta.
Shackleton said she considered the court hearing to be much more important than a simple matter of censorship.
"Tomorrow is a lot bigger than only a trial about a film. This is Indonesia on trial in front of the whole world because democracy does not tell you what to think or watch or fear," she said.
"In a true democracy the government doesn't act as a nanny. They don't give you what you think. They have to let you see the film, read the book, talk to people who lived the tragedy."
"She appreciates our invitation and she doesn't want to be paid at all for her visit here. It's very inspiring to see a 78-year old person who is still energetic and still wants to seek justice," he told AFP.
"She told us that the trial tomorrow is not about revenge, but about accountability."
At least 100,000 East Timorese lost their lives through fighting, disease and starvation during the brutal Indonesian occupation, which ended with a bloody vote for independence in 1999.