WASHINGTON — After all the swearing at, finally comes the swearing in. When Robert S. Mueller III takes the oath on Wednesday morning in the wood-paneled Room 2141 of the Rayburn House Office Building, he will answer questions for the first time since opening his special counsel investigation into President Trump and Russia more than two years ago.

But for all the anticipation, for all the fighting that it took to get to this day, many in Washington assume it will be more fizzle than sizzle. Mr. Mueller, the famously stoic prosecutor and reluctant witness, has vowed to adhere strictly to the words of his 448-page report and no more, making it unlikely that he will serve as the dramatic accuser Mr. Trump’s critics yearn to see.

“I don’t have high expectations for any additional substantive appreciation of Mr. Mueller’s investigation,” said former Senator Tom Daschle of South Dakota, who was the Democratic leader during the Senate impeachment trial of President Bill Clinton in 1999. “I think he’s going to stick to the script, and the Justice Department has told him to stick to the script, so I think it will be difficult for him to provide any more information.”

That is not to say it will be free of fireworks. Democrats will use Mr. Mueller to argue that Mr. Trump benefited from Russia’s help in the 2016 election even if investigators did not establish a criminal conspiracy and that his efforts to impede the investigation amounted to obstruction of justice even if Justice Department rules bar indictment of a sitting president. Republicans will grill the former special counsel to press their case that the entire investigation represented an illegitimate, partisan coup attempt even though Mr. Mueller himself is a lifelong Republican.