Judge Merchan told jurors that Michael Cohen is an "accomplice" to the alleged crime, so the jury cannot convict Trump on Cohen's testimony alone.

Even if they believe Michael Cohen, they need to rely on evidence corroborating his testimony, Merchan said.

Trump, at the defense table, appeared to perk up just a bit at this, looking up at Merchan as he delivered this information to the jury.

Judge Merchan told the jury that they have the ultimate authority in determining the truthfulness of a witness' testimony -- an instruction that might come in handy given the emphasis that defense lawyers placed on Michael Cohen's testimony, which they say includes lies.

"You will alone determine the truthfully and the accuracy of the testimony of each witness," Merchan said.

He added that the jury could either disregard the testimony of an untruthful witness entirely, or disregard part of it.

"There is no particular formula" to determine credibility, Merchan said.

"You may consider whether the witness has any interest in the outcome of the case," he said -- another matter that might weigh on jurors' minds.

"You may consider whether a witness did have, or did not have, a motive to lie," the judge said.

Trump's attorney had told the jury Cohen had an "axe to grind" against his former boss. Prosecutors partially conceded that Cohen had a motive, acknowledging that, to date, he is the only one who suffered any consequences from the matter at hand.

Trump, at the defense table, appeared to look over to the jury and offer a bit of a closed-mouth grin.

Judge Merchan instructed the jury that they cannot hold Trump's not testifying against him.

"Defendant is not required to prove that he is not guilty," he said.

"The defendant is not required to prove or disprove anything," said the judge.

Judge Merchan told the jury that they should not speculate about the sentence or punishment in the case.

"It will be my responsibility to impose an appropriate sentence," Merchan said.

The judge also had to advise the jury about this yesterday after defense lawyer Todd Blanche argued that the jury "cannot send somebody to prison" based on Michael Cohen's testimony.

The jury appears to be laser focused on Merchan during the charge. Half of the jurors appear to be taking notes.

As ABC News reported previously, the U.S. Secret Service spent considerable time working with New York City court and jail officials to determine what would happen if the judge, as threatened during trial, ordered former President Trump to be incarcerated for contempt -- a situation that did not come to pass.

According to an official briefed on the matter, U.S. Secret Service planners have not started the process of determining arrangements for Trump to be imprisoned at a New York state penitentiary in the event he is convicted. Because Trump is charged with nonviolent crimes, has no criminal record, and was allowed to remain free before trial, it is believed that there is no way that a conviction would lead to immediate incarceration.

If Trump is convicted, the Secret Service will spend the time prior to sentencing -- which could be months -- working with New York state prison officials to determine how imprisonment would work. An official told ABC News that potential questions could include, would USSS agents be armed inside prisons where firearms are prohibited? What prison would Trump be assigned to? Where would protection agents be stationed in relation to Trump? Those details, among many others, would have to be hammered out, the official said.

The Secret Service specifically avoided planning on a possible prison sentence during the trial so Trump and his loyalists could not accuse them of taking sides against the former president, the official said.

-Josh Margolin