WASHINGTON -- The start of public impeachment hearings unfolding in Congress marks the first time the American public can watch and listen to the witnesses whose testimony is at the centre of the Democrats' investigation.

In several hours of testimony, punctuated by occasional bickering among lawmakers, some memorable moments have emerged.

Here are some key takeaways from Wednesday's hearing featuring the first public witnesses: George Kent and William Taylor.

TRUMP PUSHED FOR BIDEN INVESTIGATION

Hearings like this one can be scripted affairs. But on Wednesday, there was one early surprise.

Taylor, the top diplomat in Ukraine, revealed for the first time that his staff members overheard the president speaking on the phone to another diplomat about investigations.

Taylor said some of his staff members were at a restaurant with Gordon Sondland, Ambassador to the European Union, on the day after the July 25 call between President Donald Trump and Ukraine's newly elected president, Volodymyr Zelenskiy.

Sondland used his cellphone to call Trump from the restaurant and the staff members could hear Trump on the phone asking about "the investigations." Taylor took that to mean investigations into the Bidens and the Burisma Group, the Ukrainian firm that had hired Hunter Biden as a director in 2014, he told lawmakers.

Sondland told the president that the Ukrainians were ready to move forward, and after the call, one of Taylor's staffers asked Sondland what Trump thought about Ukraine, he said.

"Ambassador Sondland responded that President Trump cares more about the investigations of Biden, which Giuliani was pressing for," Taylor said in his opening statement.

"I take it the import of that is that he cares more about that than he does about Ukraine?" Schiff asked Taylor.

"Yes, sir," he responded.

Taylor said he only learned about the call last Friday and didn't know about it when he appeared for a closed-door deposition with House investigators conducting the impeachment inquiry.

IS IT BRIBERY, OR A QUID PRO QUO?

The impeachment inquiry in many respects is about controlling the narrative. On Wednesday, Democrats seemed to begin to pivot, framing the actions of Trump as possible "bribery" and "extortion" rather than emphasizing a "quid pro quo."

House Intelligence Committee Chairman Adam Schiff introduced the notion of bribery into the debate when he criticized acting White House Chief of Staff Mick Mulvaney for saying that people concerned about Trump's requests that Ukraine do political investigations should "get over it," and that there is political influence in all foreign policy.

Schiff asked in his opening statement: "If we find that the President of the United States abused his power ... or if he sought to condition, coerce, extort, or bribe an ally into conducting investigations to aid his reelection campaign and did so by withholding official acts -- a White House meeting or hundreds of millions of dollars of needed military aid -- must we simply "get over it?"

A Democratic staff lawyer, Daniel Goldman, also talked of possible "extortion and bribery" -- a gradual change in wording that could preview Democrats' approach going forward.

LOFTY LANGUAGE

The credibility of witnesses is in part a function of their biographies, and Democrat chose the Taylor and Kent for theirs.

One a Vietnam War veteran and West Point graduate, the other a career foreign service officer whose family has served the country for generations -- nodded to the history of the moment with language both lofty and personal.

Their phrasing laid bare the stakes of the proceedings even as some Republicans sought to minimize them, and their intentional references to serving under presidents of both parties were aimed at pre-empting Republican attacks on them as political partisans. That didn't stop Rep. Devin Nunes, the top Republican on the House intelligence committee, from deriding the witnesses as being part of a smear campaign, "star chamber" from within the civil service.

Taylor capped his opening statement with an ode to how Americans feel in their best moments about their country: "less concerned about what language we speak, what religion if any we practice, where our parents and grandparents came from; more concerned about building a new country."

For his part, Kent singled out by first name and even nickname the impeachment witnesses who were born abroad, likening them to 20th century national security policy: Henry Kissinger and Zbigniew Brzezinski, both immigrants.

"Masha, Alex, and Fiona were born abroad before their families or they themselves personally chose to immigrate to the United States. They all made the professional choice to serve the United States as public officials, helping shape our national security policy, towards Russia in particular. And we and our national security are the better for it," Kent said.

"IT WAS CRAZY"

Crazy. Counterproductive. Illogical.

Taylor was unsparing, and colorful, in his characterization of making military aid to Ukraine contingent on the country announcing investigations into the 2016 U.S. election and into Trump's political rival Joe Biden.

He was presented with oversized images of a September text message exchange with two other envoys in which he said it would be "crazy" to not provide military assistance to the former Soviet republic for domestic political gain. Those text messages were the among the first documentary pieces of evidence to become public as part of the House impeachment inquiry, and established not only the possible contours of a quid pro quo but also laid bare diplomatic concerns about the Trump administration's dealings with Ukraine.

Asked to elaborate, Taylor said, "It was counterproductive to all of what we had been trying to do. It was illogical, it could not be explained, it was crazy."

Security assistance to Ukraine, he said, was not only critical to that country but also to America's own national interests.