The House impeachment hearings are scheduled to continue Thursday with the highly anticipated testimony from Fiona Hill, a former top National Security Council expert on Russia, and David Holmes, a State Department official.

Their scheduled appearances will follow Wednesday's marathon day of testimony, most notably from Gordon Sondland, the U.S. ambassador to the European Union, whose remarks raised eyebrows on both sides of the aisle.

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Hill is among several witnesses who have testified behind closed doors. It is believed she played a central role in a July 10 meeting at the White House in which Sondland and Acting White House Chief of Staff Mick Mulvaney allegedly told Ukrainian officials that President Trump would meet with Ukrainian President Volodymyr Zelensky -- if Zelensky would agree to investigate the Ukraine business dealings of former Vice President Joe Biden and his son Hunter Biden.

At that meeting, Hill said, former national security adviser John Bolton "immediately stiffened and ended the meeting."

Bolton "made it clear that he believed that they were making, basically, an improper arrangement to have a meeting in the White House, that they were predicating the meeting in the White House on the Ukrainians agreeing, in this case, based on the meeting on July 10, to restart investigations that had been dropped in the energy sector," Hill said.

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She added that Bolton later told her: "I am not part of whatever drug deal Sondland and Mulvaney are cooking up" and asked her to relay that message to a White House lawyer.

Hill said she reminded Sondland after the meeting of the need for proper procedures and the role of the National Security Council in talks between the U.S. and foreign leaders.

'I remember it vividly'

During his closed-door testimony, Holmes told lawmakers he was sitting in close proximity to Sondland at a restaurant in Kiev -- just one day after the highly controversial July 25 phone call between Trump and Ukraine's leader that set off the impeachment inquiry -- and overheard a phone call between Trump and Sondland, in which he heard the president ask the ambassador how the "investigation" was going.

Holmes said Trump was talking loud enough over the phone that he could hear the president say, "So, he's gonna do the investigation?" referring to Ukraine's president, Zelensky. Holmes then said he heard Sondland reply, "He's gonna do it," and tell the president that Zelensky “loves your a--" and would do "anything you ask him to."

Holmes said Sondland later told him that "the president did not 'give a sh-- about Ukraine'" and that he only cares about "big stuff" like the "Biden investigation."

"I've never seen anything like this," the State Department official reportedly said. "Someone calling the president from a mobile phone at a restaurant, and then having a conversation of this level of candor, colorful language. There's just so much about the call that was so remarkable that I remember it vividly."

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Democrats are expecting that Holmes' testimony will support the notion that Trump initiated a "quid pro quo" deal with Ukraine -- that Trump would provide military aid to the country in exchange for an investigation into the Ukraine business dealings of Biden and his son Hunter Biden.

Republicans, on the other hand, are likely to point out that Holmes previously testified that he immediately reported the phone call to William Taylor, the U.S. chargé d'affaires for Ukraine, contradicting Taylor's claim that he had learned of the July call only last month.

Fox News' Gregg Re and Alex Pappas contributed to this report.