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, Mr. 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.