From The Accidental President: Harry S. Truman and the Four Months That Changed the World, by A. J. Baime (HMH Books, 2017), Kindle pp. 140-142:
At 10:30 a.m., dozens filed into the Oval Office for Truman’s first press conference. Standing behind his desk, he greeted reporters as they pushed into the room, which quickly grew uncomfortably crowded. Regular presidential press conferences were a tradition going back to Woodrow Wilson, who on March 15, 1913, set a precedent of welcoming newspaper reporters into his office to answer questions. Roosevelt had held two a week and had elevated these meetings to high art. Wielding his cigarette holder as if conducting an orchestra, he would deliver soliloquies that would entrance his guests, while almost always failing on purpose to answer any question posed.
On April 17 the largest crowd ever assembled for a presidential press conference pushed into the Oval Office—348 men and women reporters—all aiming to size up the new chief executive. Some were forced to stand on the terrace outside the president’s office—lucky ones, because the room got exceedingly hot.
“Good morning,” Truman said, “good morning.”
“Good morning, Mr. President,” someone in the crowd said. “Will you take it sort of slow for us today, please, sir?”
“Surely, surely,” Truman said. “Anything I can do to accommodate you.”
No one in the room could help making comparisons to Roosevelt. For one thing, this president was standing up. “We all knew that Roosevelt had gone to Groton and then Harvard,” recalled White House correspondent Robert Nixon, who was getting his first crack at Truman that morning. “That [Roosevelt] came from a quite old, well-to-do family; that he moved in what is known as the best circles all of his life . . . Truman was a small town, Midwestern Missourian of farm origin . . . The contrast was in appearance, voice mannerisms, and even their attire. President Roosevelt, while a casual dresser, was very well tailored . . . Truman dressed like he had just come off of Main Street in Independence.”
The new president called for attention. “The first thing I want to do to you is to read the rules,” he said. After telling the reporters what they already knew—everything he said was background material, no direct quotes were allowed unless there was specific permission—he began by announcing that most of the Roosevelt staff would stay on, and that Matthew Connelly had been appointed his confidential secretary. Truman read a letter aloud from Mrs. Roosevelt, thanking everyone for their wishes, “which have brought great comfort and consolation to all of us.” Due to the wartime paper shortage, Mrs. Roosevelt would not be responding to all correspondence. Instead, she had asked Truman to read her thank-you letter to the press.
Truman then opened the floor. He answered questions about reciprocal trade, race relations, the wartime ban on horseracing, and the historic United Nations Conference set to open in eight days.
“Mr. President,” said one reporter in the crowd. “Will Mrs. Truman have a press conference?”
“I would rather not answer that question at this time.”
At numerous moments Truman delivered witticisms that sparked laughter in the room. The Missourian had a simple way of speaking that amused his counterparts in the press. He whittled his ideas down to the fewest words and handed them over. Unlike Roosevelt, Truman actually answered questions, and if he chose not to, he said just that.
“His first press conferences were wonderful,” noted press secretary Daniels. At the end of this first one, something happened that had never occurred in any of Roosevelt’s meetings with the press: the room erupted in spontaneous applause.