Prejean gained national prominence as a death penalty abolitionist in 1995 with the release of the movie "Dead Man Walking."
She told CNN last summer that she doesn't concern herself with a condemned inmate's guilt or innocence. It's easy to forgive the innocent, she said. It's the guilty who test our morality.
Lead defense attorney Judy Clarke
, who has kept some of the nation's most notorious killers off death row, signaled her plans during a sidebar Wednesday out of earshot of the jury. She said Prejean would be one of the final two "potential" defense witnesses in the penalty phase of the case.
Assistant U.S. Attorney William Weinreb responded that prosecutors planned to file legal papers seeking to exclude Prejean's testimony, according to a transcript purchased by CNN. The legal papers have not been made public.
A dispute over Prejean's testimony led to hours of legal wrangling behind closed doors and a decision by the judge to send the jury home at lunchtime. If she testifies at all, it will be on Monday and any testimony will likely to be very limited and focused.
The defense has been working for more than a week to convince jurors to spare the life of the 21-year-old Tsarnaev. With his late brother, Tamerlan, he set off two homemade bombs
near the finish line of the 2013 marathon; three people died in the blasts and another 17 lost limbs.
Tamerlan Tsarnaev, 26, was killed during a gun battle with police three days after the bombing.
The defense has portrayed Dzhokhar Tsarnaev, who was called Jahar, as a likeable but lost boy under the sway of an older brother obsessed with radical Islam and waging jihad. Some 40 witnesses -- including relatives brought from Russia and former teachers and classmates -- portrayed Tamerlan as overbearing and sometimes violent, but "kind" was the word used most often to describe Jahar.
Clarke has acknowledged Tsarnaev's role in the bombings, but insists they would never have happened if not for Tamerlan.
Longtime fight against executions
Now in her mid-70s, Prejean embarked on a mission three decades ago to end the death penalty based on her Catholic faith and belief in human rights. She says the Universal Declaration of Human Rights forbids torture, cruel and unusual punishment and degrading treatment.
Prejean believes executions are a form of torture, as is waiting for them.
She spends much of her time ministering to the condemned men, and they are mostly men, on death row. "You are a son of God," she tells them. "Christ is with you. What is being done to you is wrong."
Prejean was raised middle class and Catholic in Baton Rouge, Louisiana, and decided to become a nun. Her social conscience was awakened during the 1980s, and she decided to work with the poor. She found plenty of poor, black people who needed her at the St. Thomas housing project in New Orleans.
She began to correspond with an inmate at the notorious Angola State Prison, Elmo Patrick Sonnier; he had been convicted of a brutal kidnapping, rape and double murder.
He told her he was Catholic. She visited him at Angola, saw a human being and was present on the day he was executed -- April 5, 1984, in an electric chair called "Gruesome Gertie." She has witnessed five other executions since them.
She wrote two books about capital punishment, including "Dead Man Walking," which was turned into a movie starring Susan Sarandon and Sean Penn.
Was he defiant?
Weinreb also said during Wednesday's sidebar conversations that prosecutors wanted to introduce "defiant notes" Tsarnaev wrote in his hospital bed as he recovered from being shot by police during his capture.
"Your honor, two days when he lay in the bed in Beth Israel (hospital), he wrote one defiant note after another," Weinreb told O'Toole.
The defense had called a federal marshal, who testified that
Tsarnaev apologized after he was scolded for raising his middle finger at a surveillance camera in a holding cell at the courthouse.
Prosecutors had planned to show the notes to the jury if the defense argued that Tsarnaev had not been defiant while in custody.