Some are watching old video of his previous testimony. Others are closely re-reading his 448-page report. And almost all are worrying about how they'll make the most of the short time they'll have for questioning.
Robert Mueller, the Democrats know, will be tough to crack.
The stern, reticent former FBI director has said he won't answer questions beyond what is in the report on Russia's election meddling and the Trump campaign and possible obstruction of justice when he comes to Congress on July 17.
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Mueller is expected to testify in front of the Judiciary and intelligence committees for two hours each, with time split evenly between Republicans and Democrats, though that timing is still a subject of negotiations. That means Democrats will have to be efficient and targeted in their attempts to extract information from the former special counsel and spotlight what they say are his most damaging findings against President Donald Trump.
"It will not be easy," said Rhode Island Rep. David Cicilline, a Democratic member of the Judiciary Committee. He added: "We just have to be very smart about how we use the time and really give the special counsel the time to tell the story."
Cicilline says he's reading the report a second time, thoroughly, with an eye toward what he wants to ask.
Separately, a Democratic aide said staff members have been watching old videos of Mueller testifying as FBI director during the administrations of Presidents George W. Bush and Barack Obama. They're looking to see how he'll act, the aide said, and they have noticed he gives minimal commentary when answering questions. The aide was not authorized to discuss internal preparations for the hearing and requested anonymity.
Wary of their challenging witness, Democratic members of the Judiciary Committee huddled Wednesday evening to discuss strategy for questioning Mueller, along with other topics. Exactly how the hearing will be structured is still being negotiated, members said as they emerged, but Democrats are expected to divvy up the questions in a methodical way
Among the topics up for discussion as the hearing approaches: Should they work through the report step by step, or paint a general picture? Will every member be able to speak in the short time they have? And what can they do to best crystalize the findings of a report that they believe Americans haven't read or absorbed?
New York Rep. Hakeem Jeffries, a member of the panel, said before the meeting that he expects to discuss "what the team strategy is going to be as we begin an intensive phase of preparation."
Republicans seem to have given it less thought. Ohio Rep. Steve Chabot, a senior GOP member of Judiciary, said he hasn't started preparing and expects little news from the event. He said Democrats are just "chasing their tails" and are aiming to placate base voters who want to see the Democratic House majority take on the president.
"It's possible a few people could change their opinion, but overall I think it's not likely," Chabot said.
The Judiciary Committee is expected to focus on the second half of Mueller's report, which details multiple episodes in which Trump attempted to influence the investigation. Mueller said he couldn't exonerate the president on obstruction of justice.
The House's intelligence panel, which will go second, will focus on the first half of the report, which details Russian interference in the presidential election. Mueller said there wasn't enough evidence to establish a conspiracy between Russia and the Trump campaign, but detailed several contacts between the two as well as the Trump campaign's willingness to accept Russian help.
Under a deal struck with the committees, two of Mueller's deputies — James Quarles and Aaron Zebley — are expected to meet with the panels in separate closed sessions after Mueller's public hearing. But that might be in jeopardy as the Justice Department has pushed back on the arrangement, according to two people familiar with the negotiations. They requested anonymity to discuss the private talks.
The chairman of the intelligence panel, Rep. Adam Schiff, D-Calif., said Tuesday said he wouldn't discuss the details of those negotiations, but that the deputies have agreed to appear and "I have no reason to believe that will be unsuccessful."
One issue that Judiciary members are expected to focus on is whether Mueller will state whether Trump would have been charged with a crime were he not president. Jeffries said that answer could "strike to the heart of why a prosecution or recommendation to prosecute wasn't included in the report."
Mueller said at a May news conference that charging a president with a crime was "not an option" because of longstanding Justice Department policy. But Democrats want to know more about how he made that decision, and when.
It's unclear if he will go beyond his previous comments. Mueller, who was reluctant to testify at all, has been firm that he will stick to what's already in the report.
Some lawmakers say that's OK and just want to reach a broader audience of Americans who they fear have tuned out.
"This isn't a question of creating a narrative," said Florida Rep. Ted Deutch, another Democrat on the Judiciary Committee. "The narrative is already out there. It's simply highlighting what is already there."
Associated Press writer Michael Balsamo contributed to this report.