The schedule has been cleared and the popcorn readied at Evergreen Partners, a strategic communications firm in central New Jersey, where the rule for employees on Thursday morning is simple: No client talk while James B. Comey is speaking.

“We canceled meetings when we saw what time it was on,” said the firm’s president, Karen J. Kessler, who is planning a cheese-and-crackers spread by her office’s 60-inch screen. “It’s must-see TV.”

Americans do not agree on much these days. But millions are expected to pause on Thursday to take in a spectacle already being compared to other political-cultural touchstones, like the Army-McCarthy hearings and Anita Hill’s testimony about Clarence Thomas. This time, Mr. Comey, the former F.B.I. director, will dish to the Senate Intelligence Committee about President Trump, the man who fired him.

By the time details from Mr. Comey’s prepared remarks surfaced on Wednesday — revealing an anxious president pressing his F.B.I. director about a continuing investigation into his campaign’s contacts with Russia — the hearing had leapt from Beltway curiosity to required viewing, a cliffhanger episode of the nation’s real-life reality show.