This week, the public will get to hear directly from impeachment witnesses as they are likely to describe a subversion of American policy toward Ukraine by a handful of President Trump’s allies. Here is a guide to the hearings, with State Department officials George Kent and Bill Taylor up first on Wednesday.

FORMAT

The impeachment hearings follow a format that is unusual for Congress: Staff attorneys are expected to ask the bulk of the questions. Each party gets 45-minute blocks for questioning by House Permanent Select Committee on Intelligence Chairman Adam Schiff and the top Republican, Rep. Devin Nunes, or their staff.

After the staff questioning, rank-and-file committee members will be given five minutes each in a different block to question witnesses. Democratic lawmakers and aides said this will allow staff to get to the key points of the testimony and avoid a disjointed narrative with lawmakers bouncing from different topics every five minutes. It may also limit grandstanding by lawmakers who want their time in the sun.

Expect Mr. Schiff to intervene if Republicans ask questions that could identify an intelligence official whose initial whistleblower complaint against Mr. Trump first sparked the investigation. In a letter sent to colleagues, the chairman suggested it could be an ethics violation for a lawmaker to undermine the confidentiality of whistleblowers.

THEMES

Irregular diplomacy: On May 23, Mr. Trump told a trio of officials—U.S. Ambassador to the European Union Gordon Sondland, Energy Secretary Rick Perry and then-U.S. envoy Kurt Volker —that Ukraine had tried to hurt him in the 2016 election and that the officials should talk to his personal lawyer, Rudy Giuliani, about his concerns, based on interviews and depositions. The president’s marching orders set in motion what the witnesses have described as an irregular diplomatic channel through which the Trump administration used held-up military aid to Ukraine and a White House meeting to pressure that country to initiate investigations that could help the president politically. Among the episodes described at which the pressure campaign became visible: an early July meeting in Toronto with Ukrainian officials at which Mr. Kent was present and a pair of phone calls in early September involving Mr. Taylor.

Origins of Trump’s ire: The story within the story involves how Mr. Trump came to embrace the idea that Ukraine, and not Russia, had meddled in the 2016 election, an unfounded theory that some advisers tried to wave the president away from. The witnesses can introduce the public to a cast of characters, including a one-time Ukrainian prosecutor with whom the U.S. had repeatedly clashed for failing to prosecute criminals while simultaneously investigating without basis reporters, political opponents and civil-society activists. The prosecutors—figures who had lost or were at risk of losing their jobs as Ukrainian public sentiment favored reformers—were working with Mr. Giuliani as he launched a disinformation campaign, the witnesses learned from Ukrainian contacts.

No first-hand information?: From their perches at the State Department, this week’s witnesses were privy to a lot, thanks to their conversations with both Ukrainian contacts and Trump officials. But one place into which they had limited visibility: the so-called seventh floor, the home of the State Department’s leadership, including Secretary of State Mike Pompeo, and the inner sanctums at the White House, including the Oval Office and acting chief of staff Mick Mulvaney’s office.

Mr. Kent has described pushing for a strong show of support from top State Department officials for then-embattled Ambassador Marie Yovanovitch, only to hear that the ambassador to Ukraine should tweet her support for Mr. Trump, and to watch as top officials decided to plead with talk-show host Sean Hannity to stay out of the embassy-bashing. The witnesses learned of the hold on military aid when an anonymous budget official piped up on a July 18 call saying that the hold came from the president via Mr. Mulvaney. Mr. Taylor, whose notetaking skills and inclusion in conversations in the irregular diplomatic channel make him a star witness for Democrats, has said he has a “clear understanding” of a link between military aid and investigations, but that is largely based on a conversation with Mr. Sondland and a separate conversation with a National Security Council official—who passed along details of yet another conversation.

Difference from modern-day impeachment inquiries: In the inquiries of former Presidents Richard Nixon and Bill Clinton, outside investigators did much of the preliminary work identifying witnesses and deposing them.

Independent counsel Kenneth Starr conducted his probe of Mr. Clinton using a grand jury, and then he transmitted his findings to the GOP-led House Judiciary Committee. In the Watergate investigation of Mr. Nixon, special counsel Leon Jaworski also used a grand jury to collect evidence that he eventually transmitted to Congress during its impeachment inquiry.

Without an outside investigation to rely on this time, the House Intelligence Committee managed the inquiry into allegations outlined in the whistleblower report and deposed witnesses behind closed doors. Now, the panel is bringing what it has learned into public view with hearings. Lawmakers and aides expect the intelligence panel to refer its findings at some point to the House Judiciary Committee, which oversees impeachment and could refer potential articles of impeachment to the House floor.

In the case of Mr. Clinton, the Judiciary hearings were largely perfunctory, as the House relied heavily on Mr. Starr’s report. In the case of Mr. Nixon, the Democratic-controlled Senate had already held hearings on the 1972 break-in at the Watergate office building, and more than a year of investigations resulted in evidence, such as the tapes and transcripts of Mr. Nixon’s meetings in the Oval Office.

Mr. Schiff noted to Democrats one factor that is similar: During the Nixon and Clinton impeachments, the rules permitted information to stay confidential until the committees chose to release it. Last week and this week, the intelligence committee, along with the oversight and foreign-affairs panels, have released transcripts from witnesses.

Republicans argue that the process isn’t fair for the president because Mr. Trump and his counsel can’t participate in the hearings in the House Intelligence Committee. A resolution passed by the House said Democrats will allow the president and his counsel to participate when the hearings are in the Judiciary Committee, which is where the president and his counsel were granted some privileges in the past.

THE PLAYERS

Witnesses:

Bill Taylor. The acting ambassador to Ukraine is expected to testify about what he has called an “irregular, informal channel” handling Ukraine policy that included the president’s attorney, Mr. Giuliani. Mr. Taylor testified, along with others, that he believed the release of nearly $400 million in security aid, as well as a White House meeting, were contingent on Ukrainian President Volodymyr Zelensky announcing investigations.

George Kent. The senior State Department official for European and Eurasian Affairs is expected to outline his growing concern about the administration using informal diplomatic channels that circumvented normal procedures and efforts to oust Marie Yovanovitch when she was the U.S. ambassador to Ukraine.

Marie Yovanovitch. The former ambassador to Ukraine is expected to detail the effort to oust her. Democrats say that while it isn’t directly tied to the aid, the effort by the Trump administration to push her out shows how badly it wanted announcements of investigations in the region.

Lawmakers:

Rep. Adam Schiff. Expect the California Democrat who is leading the probe to take a forceful role. He will deliver an opening statement and then plans to defer to the committee’s attorneys for much of the questioning, but he will discipline anyone attempting to get identifying information about the whistleblower into the public record.

Rep. Jim Himes. The Connecticut Democrat is a senior member of the Intelligence panel and has said he understands that Democrats need to do a better job explaining why they want to impeach the president over the Ukraine matter. Expect his questioning to attempt that.

Rep. Devin Nunes. As the top Republican on the Intelligence Committee, the Californian is the only GOP lawmaker allowed to deliver an opening statement and who can ask questions throughout the Republican time for questioning. Transcripts show that in the previous depositions, he often deferred to the attorneys.

Rep. Jim Jordan. The Ohio Republican is a fierce defender of the president. The top Republican on the Oversight Committee, he was placed on the Intelligence Committee for the public phase of impeachment.

Key attorneys:

Daniel Goldman. The director of investigations for the House Intelligence Committee, Mr. Goldman was a federal prosecutor in New York for a decade. He was the deputy chief of the Organized Crime Unit for the Southern District of New York and has led the depositions for Democrats during the impeachment inquiry. He was a legal analyst for NBC and MSNBC before joining the House Intelligence Committee.

Daniel Noble. Another former federal prosecutor, Mr. Noble has helped lead many of the depositions for House Democrats during the impeachment inquiry. Mr. Noble, now a senior counsel to the House Intelligence Committee, worked in the Office of Complex Frauds and Cyber Crime in the Southern District of New York.

Stephen Castor. A member of the Republican Oversight committee staff since 2005, Mr. Castor has been part of numerous high-profile investigations, including the 2012 terrorist attack that killed four Americans in Benghazi, Libya, and the failed gun-trafficking operation called Fast and Furious. He may attempt to get more information about the whistleblower and to poke holes in witness testimony.

—Andrew Duehren contributed to this article.