WASHINGTON — The routine was always the same. President Trump’s lawyers would drive to heavily secured offices near the National Mall, surrender their cellphones, head into a windowless conference room and resume tense negotiations over whether the special counsel, Robert S. Mueller III, would interview Mr. Trump.

But Mr. Mueller was not always there. Instead, the lawyers tangled with a team of prosecutors, including a little known but formidable adversary: Andrew D. Goldstein, 44, a former Time magazine reporter who is now a lead prosecutor for Mr. Mueller in the investigation into whether the president obstructed justice.

Mr. Mueller is often portrayed as the omnipotent fact-gatherer, but it is Mr. Goldstein who has a much more involved, day-to-day role in one of the central lines of investigation.

Mr. Goldstein, the lone prosecutor in Mr. Mueller’s office who came directly from a corruption unit at the Justice Department, has conducted every major interview of the president’s advisers. He questioned Donald F. McGahn II, Mr. Trump’s former White House counsel, and Michael D. Cohen, Mr. Trump’s former fixer and lawyer, for dozens of hours. He signed Mr. Cohen’s plea agreement. He conducted grand jury questioning of associates of Roger J. Stone Jr., the former adviser to Mr. Trump who was indicted last month.