Why is this different from normal hearing procedures?
Starting with 45-minute segments will allow each side to develop a narrative that would be more difficult to build in the usual five-minute format. The five-minute limit can be frustrating when witnesses are argumentative and lawmakers do not have time to follow up.
This happened in a contentious September House Judiciary Committee hearing featuring Corey Lewandowski, who ran Trump's campaign for part of 2016. Lewandowski argued with the panel's chairman, Representative Jerry Nadler, and Republicans and Democrats then argued over whether Nadler had used up his questioning time.
How do Democrats plan to make their case?
The Democrats will ask three diplomats to discuss their understanding of events before and after a July 25 phone call between Trump and Zelenskiy. According to a rough White House transcript of that call, the president pressed Zelenskiy to investigate an unsubstantiated conspiracy theory involving the 2016 election and a Democratic Party computer server, as well as a Ukrainian energy company in which Hunter Biden had been a board member.
Trump's obstruction of Congress
Democrats are also expected to try to use the hearings to show Trump obstructed Congress, the basis of another possible article of impeachment, by detailing how he has blocked some witnesses from appearing and otherwise refused to cooperate with their probe.
However, Trump and/or his lawyer would be allowed to attend later hearings before the House Judiciary Committee, which will debate what, if any, articles of impeachment should be filed and sent to the floor for a vote.
Who are the witnesses?
The top US diplomat in Ukraine, William Taylor, is considered a critical witness to the case against Trump. Taylor was upset to find out that security aid to Ukraine, as well as a White House meeting between Trump and Zelenskiy, had been delayed for political reasons.
"It's crazy to withhold security assistance for help with a political campaign," Taylor wrote earlier this year in a text message released by House investigators. Another senior US diplomat, George Kent, will appear with Taylor at Wednesday's hearing. Kent said in closed-door testimony that he had been alarmed by efforts by Giuliani and others to pressure Ukraine to accede to Trump's demands.
Yovanovitch to testify on Friday
Former US ambassador to Ukraine Marie Yovanovitch will testify on Friday. She has said she was ousted from her post after she came under attack by Giuliani, whose associates "may well have believed that their personal financial ambitions were stymied by our anti-corruption policy in Ukraine."
Republicans want their own witnesses
Republicans have requested their own witnesses, including Biden's son and the whistleblower whose complaint triggered the impeachment enquiry into Trump. Democrats can veto Republicans' witnesses and Schiff suggested he may do so, saying he did not want the hearings to become an investigation of the Bidens or to facilitate retaliation against the whistleblower.
How do Republicans plan to respond?
Republicans have painted the Democratic-led enquiry as a partisan exercise, with Nunes saying the Democrats were pursuing a "sham impeachment process" that has mistreated Trump. They will seek to provide a different narrative for the millions of Americans expected to watch the hearings, minimising Trump's role in events and attempting to cast doubt on witness testimony.
Republicans are already attacking the Democratic witnesses, saying that Yovanovitch's recall as ambassador was a side issue and that other witnesses' knowledge of key events was largely third-hand.
Impeachment an electorate assault?
Republicans can also be expected to argue that Ukrainian officials did not feel pressured because they did not even know the $391 million in security aid had been held up at the time Trump asked them last July for a "favour." They have also emphasized that the Ukrainians never announced the investigations Trump wanted and that Zelenskiy said he did not feel "pushed" by Trump.
Some Republicans may also follow the lead of Republican Representative Michael Turner, a member of the intelligence committee who said in September that Trump's telephone conversation with Zelenskiy was "not OK," but impeachment would be an "assault" on the electorate.