The first public presidential impeachment hearings in over 20 years continue on Tuesday with lawmakers' busiest day yet, as they're set to hear testimony from four witnesses — three of whom were listening in on the July 25th phone call between President Donald Trump and Ukrainian President Volodymyr Zelenskiy.

Two of the three, National Security Council staffer Lt. Col. Alex Vindman and Jennifer Williams, an aide to Vice President Mike Pence, thought the call was troubling. The third, former NSC staffer Tim Morrison, said at his closed-door deposition that he didn't think there was anything illegal about the call, but recommended it be secured for fear it would leak.

Republicans on the House Intelligence Committee had asked that Morrison and the fourth of the day's witnesses, former special envoy to Ukraine Kurt Volker, be called to testify publicly. Both have defended the president — but both have also provided information corroborating Democrats' assertions that Trump was withholding aid to Ukraine to force its president to announce an investigation into former Vice President Joe Biden's son Hunter.

When does the hearing start?
Round 1, featuring Vindman and Williams, is scheduled to begin at 9 a.m. ET.

Round 2, with Volker and Morrison, is scheduled to begin at approximately 2:30 p.m. ET.

Who's testifying?
Vindman, a decorated lieutenant colonel in the Army and the top Ukraine expert on the NSC. In his closed-door deposition, he testified that some details had been omitted from the White House reconstruction of the call between Trump and Zelenskiy, including that Zelenskiy had mentioned Burisma, the company Biden worked for.

Vindman said he was concerned about the contents of the phone call, and mentioned his concerns to the NSC's lawyer.

"I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government's support of Ukraine," Vindman testified.

Williams, the Pence adviser, told impeachment investigators that she thought it was "unusual and inappropriate" that Trump had asked Zelenskiy to investigate the Bidens and whether the Democratic National Committee's computer server was in Ukraine.

Trump fired back at Williams on Sunday, tweeting, "Tell Jennifer Williams, whoever that is, to read BOTH transcripts of the presidential calls, & see the just released ststement [sic] from Ukraine. Then she should meet with the other Never Trumpers, who I don't know & mostly never even heard of, & work out a better presidential attack!"

Volker, a longtime diplomat who resigned his post as envoy to Ukraine after being named in a whistleblower report, has testified that he thought Trump was being fed "a negative narrative" on Ukraine by his personal lawyer, Rudy Giuliani. He also said he did not believe there was a connection between the aid and the president's desire for "investigations" — but he did acknowledge a link between investigations and a White House visit for Zelenskiy.

Morrison, who resigned from NSC last month, told investigators he thought there was "nothing illegal" about Trump's call with Zelenskiy, but was worried that if details of conversation leaked out, they could harm the U.S.-Ukraine relationship. He also testified that Gordon Sondland, the U.S. ambassador to the European Union, told a Ukrainian official on Sept. 1 that what "could help them move the aid was if the prosecutor general would go to the mic and announce that he was opening the Burisma investigation."

Who's doing the questioning?
Committee chair Adam Schiff, D-Calif., and ranking member Devin Nunes, R-Calif., will have 45 minutes each to question witnesses at the beginning of each hearing. They can — and are expected to — delegate the bulk of the questioning to their committee lawyers instead. Daniel Goldman, a former federal prosecutor, is the lawyer for the Democrats and Steve Castor, the minority counsel for the House Oversight committee, is the lawyer for the Republicans.

Schiff will determine if there is a need for additional questioning by the lawyers at the end of the 90 minutes, and if not, they move on to lawmaker questions. That resembles a traditional congressional hearing, with all lawmakers on the panel getting five minutes each to ask questions.

Who's on the committee?
There are 13 Democrats and nine Republicans. The Democrats are Schiff, Eric Swalwell of California; Joaquin Castro of Texas; Jim Himes of Connecticut; Terri Sewell of Alabama; Andre Carson of Indiana; Jackie Speier of California; Mike Quigley of Illinois; Denny Heck of Washington; Peter Welch of Vermont; Sean Patrick Maloney of New York; Val Demings of Florida and Raja Krishnamoorthi of Illinois.

The Republicans are Nunes; John Ratcliffe of Texas; Jim Jordan, Michael Turner and Brad Wenstrup of Ohio; Chris Stewart of Utah; Elise Stefanik of New York; and Mike Conaway and sometimes-Trump critic Will Hurd of Texas.

The hearing is set to take place in the House Ways and Means Committee hearing room because the Intelligence Committee doesn't have its own hearing room.

What to expect from each side
Democrats are expected to press Vindman, Williams and Morrison about how unusual they thought the Trump-Zelenskiy call was, and get them to expound on their concerns. With Volker, Democrats will likely ask him more about his dealings with Giuliani and Sondland, and whether testimony from other witnesses has changed his mind about whether he thought the almost $400 million in aid was being tied to Trump's desire for investigations.

Republicans are expected to press Vindman and Williams on their lack of interactions with the president, while focusing on Morrison and Volker's past statements about the president not doing anything wrong.

How long will the hearing run?
To be determined. The committee expects the first round of testimony to wrap up between 1:30 and 2:30 p.m. ET, and the second round to wrap up between 6 and 8 p.m. Lawmakers will have to take a break at some point to vote for a continuing resolution to keep the government open and funded until Dec. 20.

Where can you watch?
NBC News will air a special report beginning at 9 a.m. ET led by "NBC Nightly News" anchor Lester Holt, NBC News chief legal correspondent Savannah Guthrie and NBC News political director Chuck Todd.

MSNBC will have special coverage beginning at 9 a.m. ET anchored by "Deadline: White House" host Nicolle Wallace.

The hearings will be streamed live on NBC News NOW, NBCNews.com and MSNBC.com. NBCnews.com will also feature a live blog with contributors from across NBC News with news, fact checks and analysis. The coverage will be collected at NBCNews.com/impeachment.

Are more public hearings scheduled?
The Intelligence Committee has hearings scheduled for Wednesday and Thursday. The committee will hear testimony from Sondland on Wednesday morning, and then testimony from Laura Cooper, the deputy assistant secretary of defense for Russian, Ukrainian, and Eurasian Affairs and David Hale, the undersecretary of state for political affairs, later in the day.

On Thursday, the panel will hear from Fiona Hill, the former NSC senior director for Europe and Russia who testified that Sondland had told Ukrainian officials they needed to proceed with "investigations" to line up a White House visit for Ukraine's president. David Holmes, the U.S. Embassy in Ukraine official who overheard a July phone call between Sondland and Trump where the president was demanding "investigations," will testify alongside Hill, the committee announced Monday.