Nine thousand prospective jurors - the largest jury pool in US history - begin arriving today for the Batman cinema shooting trial.

Whittling those numbers down to the 12 jurors and 12 reserves is expected to take months, and the trial at the court in Centennial, in Denver, Colorado, could last until October.

At first glance, the case seems simple, as James Holmes has confessed to the attack that left 12 dead and 70 injured.

But jurors must decide whether he was insane when he barged into a packed cinema, clad in combat gear, and opened fire on movie-goers in July 2012.

Experts say it is rare to have a mass shooter appear in court to face charges - many are either killed by police or commit suicide.

Alan Tuerkheimer, a Chicago-based jury consultant, said: "The public is going to get an insight into the mind of a killer who says he doesn't know right from wrong. It is really rare."

In the two-and-a-half years since the shooting, the case has sparked an emotionally charged debate, with Holmes's parents begging for a plea deal that would save his life while many survivors and family members of victims have demanded that he be put to death.

Holmes, 27, was arrested as he stripped off his combat gear in the car park of the Century 16 cinema in Aurora after he opened fire at the midnight showing of a new Batman movie. He later pleaded not guilty by reason of insanity to multiple counts of first-degree murder and attempted murder.

Under Colorado law, defendants are not legally liable for their acts if their minds are so "diseased" that they cannot distinguish right from wrong.

Part of the reason the case has dragged on so long is the battle over whether that standard applies to Holmes.

Holmes's sanity was evaluated by a state psychiatrist but the results were not made public. Prosecutors and defence lawyers remain under a long-running gag order, and court documents detailing the issue have stayed under seal.

Survivors of the attack and family members of victims have had a long time to get ready for a trial.

Marcus Weaver, who was shot in the arm and whose friend Rebecca Wingo died in the attack, said: "We've all been to therapists and have talked to our families and have our support groups, so we're prepared."

It could take until June to find the jurors and reserves who were not biased by the widespread news coverage of the shooting. Equally challenging will be finding jurors who were not personally affected by the attack.

During the selection process, Holmes's lawyers will focus on picking jurors who are morally opposed to capital punishment, even as prosecutors fight to ensure those on the panel are "death-penalty eligible", meaning they would be open to executing Holmes.