David Pecker, who once called Donald Trump "a personal friend of mine," flashed a big smile as he took the stand as the trial's first witness, belying the gravity of the moment.

Pecker cackled loudly into the microphone, jolting the room, when prosecutor Josh Steinglass, asked him about his various phone numbers that he struggled to remember.

Pecker, 72, was the publisher of the National Enquirer but prosecutors said he was "acting as a co-conspirator" in helping buy and bury damaging stories about Trump, including a doorman's false claim that Trump had fathered a love child and a Playboy model's claim of a sexual relationship with Trump, who has denied both allegations.

Trump, who once said Pecker would make a "brilliant" choice as editor of Time magazine, listened while leaning forward in his chair, arms crossed on the table, an unhappy look on his face.

Pecker testified that he had final say whether to publish any story involving a famous person.

"I had the final say of the celebrity side of the magazine," Pecker said. "We used checkbook journalism. We paid for stories."

Pecker is testifying pursuant to a subpoena. He has also secured a non-prosecution agreement with the Manhattan district attorney's office.

Prosecutors have called former American Media Inc. executive David Pecker as their first witness.

The DA alleges that Pecker, who oversaw the National Enquirer, engaged in a conspiracy with Trump and Trump's then-attorney Michael Cohen to help influence the 2016 election by killing negative stories about Trump.

In his opening statement, defense attorney Todd Blanche sought to eviscerate Michael Cohen's credibility, saying Cohen is obsessed with Donald Trump, has a desire to see Trump incarcerated and has a propensity to lie.

"He has a goal, an obsession, with getting Trump. I submit to you he cannot be trusted," Blanche said.

On Sunday night, Cohen publicly posted online that he had a "mental excitement about this trial" and the testimony he would deliver, Blanche said.

"His entire financial livelihood depends on President Trump's destruction," Blanche said. "You cannot make a serious decision about President Trump by relying on the words of Michael Cohen."

"I have a spoiler alert," defense attorney Todd Blanche told jurors during his opening statement. "There is nothing wrong with trying to influence an election. It's called democracy."

Amid frequent objections from prosecutors, Blanche argued that the Manhattan district attorney has attempted to make the payments and non-disclosure agreements between Trump and Stormy Daniels "sinister" to the jury.

Judge Merchan had to interrupt Blanche's opening after multiple objections from prosecutors, then he met the parties at a sidebar conference, after which he struck a line from Blanche's opening.

"There is nothing illegal about entering into a non-disclosure agreement. Period," Blanche restated after the portion of his opening was struck from the record.

Blanche's opening has come off more casual and off-the-cuff than the state's opening, with Blanche improvising and posing hypotheticals to argue that accountants at the Trump Organization did not run the invoices by Trump as he was "running the country."

"'Hey, we got this invoice. I know we are trying to cover it up here,'" Blanche said sarcastically about how prosecutors described how accountants received invoices from Cohen. "Absolutely not."

According to Blanche, Trump was unaware of how the invoices were processed by his employees.

"President Trump has nothing to do -- nothing to do -- with the invoice, with the check being generated, or with the entry on the ledger," Blanche said, arguing that Trump was busy "in the White House while he was running the country."

"The reality is that President Trump is not on the hook -- criminally responsible -- for something Michael Cohen might have done years after the fact. The evidence will prove otherwise," Blanche said.

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.