Roberts spent hours reading questions - or statements disguised as questions - that came from senators and were directed to the House impeachment managers and members of Trump's defense team. Under Senate rules, inquiries must be written and passed through the chief justice.
He read them without any display of partiality or emotion, even when they were designed to make pointed attacks on the president or impugn the motives of House Democrats. He even read questions relating to his own powers, or Supreme Court precedent, without giving any indication of what he thought the answer should be.
Wednesday marked a new phase of the impeachment inquiry, one more likely to reveal what Roberts believes it means for the chief justice of the United States to "preside" over an impeachment trial of the president. He displayed no inclination to play a more active role.
It is largely uncharted territory - two chief justices in the nation's history have been put in such a position. There is disagreement among those who have studied impeachment over such basic questions as whether he can break a tie vote, much less call witnesses on his own volition.
Roberts has been "less of a force than some people expected or hoped for," said Michael Gerhardt, a University of North Carolina law professor who is an expert on impeachment and testified for congressional Democrats as part of their impeachment inquiry.
Things could change.
"This is the part where Roberts the judge will come into play," Gerhardt said.
So far, Roberts has been more like a substitute teacher. He leads the Senate in the Pledge of Allegiance, takes his cues from the body's leaders about schedule and somewhat reluctantly wields the small, hourglass shaped ivory gavel that comes with presiding over the Senate. (He doesn't use one when he's at the Supreme Court.)
Roberts's timekeeping has been meticulous - he politely cut off lawyers Wednesday when they reached the five-minute mark - but his critics have not been impressed.
"So far, I think he has failed to ensure an impartial, fair trial," said Brian Fallon, executive director of the liberal organization Demand Justice. He said Roberts could have done more to maintain decorum in the chamber, where some senators folded paper airplanes and fussed with fidget spinners as House managers took turns at a marathon presentation.
"There was no effort to enforce the Senate's own rules" on decorum, Fallon said.
Roberts's only substantive intervention came in the early morning hours of the first day, when he criticized the House managers and Trump's lawyers for overheated rhetoric as the Senate debated how the process should proceed.
"I think it is appropriate at this point for me to admonish both the House managers and president's counsel in equal terms to remember that they are addressing the world's greatest deliberative body," Roberts said. The rebuke was reportedly prompted by a complaint from Sen. Susan Collins, R-Maine, who has become a focus ahead of a vote on whether to call additional witnesses.
Some have criticized Roberts for not policing the presentations of the lawyers for inaccuracies or overblown statements. Some Democrats, for instance, have said Roberts should have challenged assertions from Trump's lawyers that the House managers have presented no "evidence" of the president's wrongdoing.
Fallon alluded to Roberts's well-known metaphor for a judge's impartiality. "To be an umpire, you have to call balls and strikes," he said.
Fallon said a poll conducted for his organization showed that 67% of respondents said Roberts has an obligation to ensure that the rules of the impeachment trial are fair, rather than leaving it to the Senate. They also want witnesses to be called.
On Wednesday, Roberts continued to play it straight. He appeared not to edit the questions in any way, to question premises or to point out what might be inaccuracies. At times, he might have been reading a textbook, or a grocery list.
One question he read, directed to the House managers, concerned his own power.
"Some have claimed that subpoenaing witnesses or documents would unnecessarily prolong this trial. Isn't it true that depositions of the three witnesses in the [President Bill] Clinton trial were completed in only one day each? And isn't it true that the chief justice, as presiding officer in this trial, has the authority to resolve any claims of privilege or other witness issues without any delay?"
Rep. Hakeem Jeffries, D-N.Y., replied: "Mr. Chief Justice, the answer is yes."
Actually, the answer is disputed, at least the part about the chief justice. One of Trump's lawyers, Jay Sekulow, said the idea that the presiding officer could on his own make such decisions would be "quite a step."
Other questions from senators seemed almost designed to put into the chief justice's mouth the most explosive charges about former vice president Joe Biden, his son Hunter or some of the most controversial comments from Trump.
Reading one question from Sen. Kamala Harris, D-Calif., meant that Roberts repeated a line from a 2005 tape in which Trump talked about grabbing women by the genitals. " 'When you're a star, they let you do it,' " said Roberts, by way of Harris, who was quoting Trump's remarks.
The liberals who have criticized Roberts also realize he might be their best hope of prolonging the impeachment trial - by subpoenaing documents and witnesses - in a Republican-majority Senate where many have said they have heard enough.
In an opinion piece in The New York Times, former Obama acting solicitor general Neal Katyal, Georgetown University law professor Joshua Geltzer and former congressman Mickey Edwards argued that Roberts on his own could make such decisions.
"This isn't a matter of Democrats needing four 'moderate' Republicans to vote for subpoenas and witnesses, as the Trump lawyers have been claiming," the trio wrote. "Rather, the impeachment rules, like all trial systems, put a large thumb on the scale of issuing subpoenas and place that power within the authority of the judge, in this case the chief justice."
That view is an outlier, though. If an agreement on subpoenas cannot be reached in negotiations between Senate Majority Leader Mitch McConnell, R-Ky., and Minority Leader Sen. Chuck Schumer, D-N.Y., there is an open question about Roberts's power in the case of an issue coming to the floor in the form of a motion from Democrats.
The Constitution says that in presidential impeachment proceedings, the chief justice presides over the Senate, a role that usually is filled by the vice president. Walter Dellinger, a longtime Supreme Court advocate and official in the Clinton Justice Department, said that means the chief justice has the power to break ties, just as the vice president normally does.
"If the Senate was split 50-50, it would be incumbent on the presiding officer to decide," Dellinger said.
Gerhardt and others disagree, saying the Senate would have to specifically give the chief justice such a power. The question never arose for Chief Justice William Rehnquist during Clinton's impeachment trial.
Chief Justice Salmon Chase did break two ties during the 1868 trial of President Andrew Johnson. But Gerhardt said Senate rules were different, as was Chase's activist view of his role.
"Chase had the [Senate] rule, and Chase had the chutzpah," Gerhardt said.
Most said Roberts's definition of impartiality means deferring to the Senate, even on a motion that would fail if the members are evenly divided.
"If there is a tie vote, I will bet the farm that Roberts will not vote," Alan Frumin, a former Senate parliamentarian, said on CNN.
Caroline Fredrickson, former president of the liberal American Constitution Society, said it would make a difference if Roberts made decisions, then left it to the Senate about whether to overrule him.
For some Republican senators, "especially those facing difficult reelection campaigns, voting to overturn a decision by the chief justice might be a daunting prospect - and rightfully so," Frederickson wrote in an opinion piece for The Washington Post.
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