Donald Trump faced forward and did not appear to make eye contact with any jurors as they entered the courtroom and took their seats in the jury box.

Before any of the lawyers in the case could speak a word, Judge Merchan launched into a lengthy speech outlining how the trial will work.

"We are about to begin the trial of People of the State of New York v. Donald Trump," Merchan told the 12 jurors and six alternates.

Merchan emphasized that the burden of proof rests on the prosecutors and that jurors should presume that Trump is innocent. A guilty verdict requires that each juror determines that the state proved their case beyond a reasonable doubt, Merchan said.

"The defendant is presumed to be innocent," Merchan said. "It is not sufficient to prove that the defendant is probably guilty."

Merchan attempted to set expectations for the jurors, only two of which have ever served on a jury before. For example, Merchan told the jurors not to expect the lawyers to launch into lengthy speeches outside of the opening and closing statements.

"That happens in TV and in movies, but it doesn't happen in real trials," Merchan said.

Judge Juan Merchan ruled that if Trump takes the stand, prosecutors can question him about a number of previous legal issues -- but the judge limited the scope of the cases and the extent to which prosecutors can question him about the facts of those cases.

The ruling is a mixed bag for Trump, who had sought to entirely block questioning on these previous issues if he takes the stand.

Judge Merchan ruled that Trump can be questioned by the DA's office on six determinations from four previous proceedings, including aspects of his New York civil fraud case and the gag order violations there, as well as both verdicts in the E. Jean Carroll cases and the 2018 Trump Foundation case.

Prosecutors had originally asked to question Trump about six different proceedings with 13 total determinations.

Merchan said with his ruling, he has "greatly curtailed" how much prosecutors can discuss the underlying facts of those cases.

"The court cautions the defendant that this Sandoval ruling is a shield, not a sword," Merchan said.

Prosecutors told Judge Merchan that they need 40 minutes for their opening statements.

Defense attorneys told the judge they need 25 minutes.

The judge also announced that court will break at 12:30 p.m. ET today, after a juror had a toothache and got an emergency appointment this afternoon.

Court had already been scheduled to end early today, at 2 p.m. ET, due to the Passover holiday.

Judge Juan Merchan announced there is an issue with Juror No. 9 -- who, according to Merchan, "was concerned about media attention" of the case. According to Merchan, the juror "wasn't 100% sure" they could serve.

Merchan said they would speak to the juror in chambers to "find out what the issue is and see if this juror can continue to serve."

After a brief sidebar, the judge announced: “Juror No. 9 is going to remain with us.”

There are six alternate jurors seated in case any of the 12 jurors cannot serve.

The U.S. Secret Service has held meetings and started planning for what to do if former President Trump were to be held in contempt and Judge Juan Merchan opted to send him to short-term confinement, officials familiar with the situation tell ABC News.

Prosecutors said at this point they are seeking a fine.

"We are not yet seeking an incarceratory penalty," assistant district attorney Chris Conroy said, "But the defendant seems to be angling for that."

Officials do not necessarily believe Merchan would put Trump in a holding cell in the courthouse but they are planning for contingencies, the officials said.

There have be no discussion about what to do if Trump is convicted and sentenced to prison, they said.

"Under federal law, the United States Secret Service must provide protection for current government leaders, former Presidents and First Ladies, visiting heads of state and other individuals designated by the President of the United States," the Secret Service said in an official statement. "For all settings around the world, we study locations and develop comprehensive and layered protective models that incorporate state of the art technology, protective intelligence and advanced security tactics to safeguard our protectees. Beyond that, we do not comment on specific protective operations."

-Josh Margolin and Luke Barr