BOSTON >> He swaggers into the courtroom. Freed of handcuffs and leg manacles, he occasionally taps the witness box on his way to the defense table, where he takes his place between two lawyers. During most of the proceedings, he slouches in his chair like a bored student, picking at his goatee, his mind somewhere else.

Dzhokhar Tsarnaev, 21, one of the world’s most notorious criminal defendants, could face the death penalty on charges that he blew up spectators at the Boston Marathon almost two years ago. But his demeanor in court here is aloof.

He is almost as inscrutable now as he was at the outset of jury selection in January. And it is virtually impossible to tell from observing him what kind of a witness he might make if he took the stand in his own defense.

The trial is barreling toward that moment of decision. While it is rare for defendants in criminal trials to take the stand, except when arguing that they acted in self-defense, his lawyers could decide that they have nothing to lose. Technically, they could call him to the stand any time after the prosecution rests, which was expected to occur Monday.

And if he is called, Tsarnaev’s testimony, along with his attitude and body language, could play a major role in determining whether he spends the rest of his life in prison or is condemned to death, if he is convicted.

Legal experts say the question of whether to put him on the stand is one that his lawyers are almost certainly asking themselves every time they talk with him.

“They’ve probably rehearsed a cross with him to see how it would go,” said Michael Cassidy, a former state prosecutor and now a professor of law at Boston College.

“They might not have called it a rehearsal, or told him about it, but that’s what they’re doing,” he said. “In the back of their minds, they are constantly asking themselves: How will he be perceived if he takes the stand? How does he react when he’s asked a question, when he’s challenged by authority, when an inconsistency is brought to his attention?”

Whether to put a defendant on the stand can be one of the most perilous decisions for the defense, especially in a capital case. Either choice carries risks. A defendant can just be a bad witness, especially under cross-examination, but even one who is prepared and articulate can rub the jury the wrong way. But if a defendant does not testify, jurors can become suspicious. Also, they like to see remorse, lawyers say, and almost no one can express it better than the accused.

Those who do take the stand – the final decision is up to the defendant – usually feel they have little to lose. Bruno Richard Hauptmann, who was charged with the 1932 kidnapping and murder of Charles A. Lindbergh’s baby, took the stand and was widely seen as a terrible witness; he was convicted and executed.

Timothy J. McVeigh, the Oklahoma City bomber, who killed 168 people, did not testify in his 1997 trial. His lawyer, Stephen Jones, said in an interview that he had seriously considered putting McVeigh on the stand but dropped the idea.

“Most jurors are not going to accept the mindset or philosophy of defendants in a case where innocent people have lost their lives, the types you have in Oklahoma City and in Boston,” Jones said. McVeigh was convicted and executed by lethal injection in 2001.

Tsarnaev’s lawyers have already acknowledged that their client participated in the 2013 marathon bombings, which killed three people and injured 260 others. Their goal is to save him from the death penalty. To that end, they have argued that he was under the malevolent sway of his older brother, now dead, whom they have cast as a violent Islamic extremist; the prosecution says he was a full partner with his brother and had murder in his heart.

Because of the admission of guilt, the verdict in this first phase of the trial seems a foregone conclusion: The jury will almost certainly find Tsarnaev guilty of most, if not all, of the 30 counts against him. Several legal experts have said the chances of his taking the stand in this phase are near zero, mainly because he would needlessly open himself up to cross-examination.

“It would just be a platform for the prosecutor to lead him back through their case,” said Jules Epstein, a longtime criminal defense lawyer who has worked on death penalty cases and teaches at Widener University School of Law in Delaware.

“And,” he added, “the prosecutor would say things like, ’While the body lay there bleeding, you were eating nachos.’”

But whether Tsarnaev testifies in the second phase, in which the same jury will decide whether to sentence him to life in prison or death, may be a slightly more open question.

The sentencing phase, which could start soon since Tsarnaev’s lawyers are unlikely to put on much of a case in the guilt phase, is the defense’s chance to explain why its client should not be executed. The jurors have heard weeks of testimony from other people about the victims and the brothers’ deeds; in the sentencing phase, they will hear almost exclusively about Tsarnaev.

They have seen some of his own words, like his breezy posts on Twitter and his risumi, in which he put his best foot forward: “Responsible, hard worker, great swimmer, social, nice, can enforce rules, very enthusiastic, reliable, people person, certified life guard, first aid, CPR.”

And they saw his explanation, written three months before the bombings, for his failing grades at the University of Massachusetts Dartmouth. “This year I lost too many of my loved relatives,” he wrote, referring not to their deaths but to their leaving the Cambridge apartment that he shared with his brother. “I was unable to cope with the stress and maintain school work.”

But they also saw what Tsarnaev had scrawled inside the bullet-riddled boat where he was captured: “The U.S. Government is killing our innocent civilians,” he wrote. “I can’t stand to see such evil go unpunished.”

And they saw extremist materials on his computer, including instructions on “How to Make a Bomb in the Kitchen of Your Mom.”

The jurors have not heard his voice. Nor has the public, as he has been barred from giving interviews. He spoke briefly in court in 2013 after the bombings, but he used an accent that his friends said they did not recognize.

During a shootout with the police and during his capture, Tsarnaev suffered multiple gunshot wounds. One bullet fractured the base of his skull, his lawyers have said in court filings, and another most “likely caused traumatic brain injury.” His left eye had to be sutured shut and his jaw wired closed, and injuries to his left ear left him unable to hear. A scar remains visible down the left side of his neck, and the left side of his face appears droopy.

“There’s some speculation that he might have suffered some type of injury to his voice from the shooting,” said Daniel S. Medwed, a law professor at Northeastern University.

Legal experts said that could be one possible reason to put him on the stand – for sympathy.

“If he were to come across as injured,” Medwed said, “if they could show that he was a little boy trapped like an animal in this vessel, that could trigger some sympathy.”

But, Medwed added, any small benefit that the defense might derive from sympathy would probably be outweighed by the risks that come with cross-examination.

Putting Tsarnaev on the stand could also allow him to show remorse – “one of the most powerful mitigating factors,” said Epstein, the criminal defense lawyer.

He said it might be better, however, to suggest regret not directly through Tsarnaev but rather through a third party, perhaps someone to whom he had written a letter, because some criminal defendants at this age have a hard time articulating remorse. “That’s not part of their vocabulary,” Epstein said.

Cassidy of Boston College said Tsarnaev’s lawyers were probably in deep discussion over how effectively he could express remorse and debating whether doing so would save their client.

“If he could say he regretted his actions, say he wished the whole thing hadn’t happened, maybe weep, it might cause at least one juror to say he deserves to be spared,” Cassidy said.

And that would be all that he would need: Without a unanimous jury voting for death, Tsarnaev would be allowed to live.