The same is likely true for Trump. The President's lawyers are having ongoing preliminary discussions about an interview with Mueller's team, according to two sources familiar with the matter. While there has not been a formal request for the President to be interviewed, there have been talks that have included setting boundaries, the sources said.
"At the end of the day, I believe the courts would authorize Mueller to require by subpoena the president's testimony," said Robert Bennett, the lawyer who represented then-President Bill Clinton in his attempt to avoid a lawsuit from Paula Jones.
Clinton lost his fight at the Supreme Court in 1997. "No man is above the law," Bennett added, paraphrasing a key part of the Supreme Court's unanimous decision, which said the President wasn't immune from civil litigation.
While Trump's legal team may not be able to avoid having him give testimony, they may try to delay or limit how the President speaks in the Mueller case, according to lawyers who've followed this situation and others like it.
Trump could plead the Fifth Amendment right to avoid self-incrimination -- though that carries its own political risk of giving the impression, rightly or wrongly, that the President has something to hide. A more typical route for the White House to withhold information would be through claiming executive privilege, a special privilege of the President that allows him to keep certain documents and testimony about national security and policy deliberations secret.
But even historical claims of executive privilege were fraught once a special prosecutor and grand jury got involved. The Clinton administration offers a clear and unfavorable road map for Trump's team to follow.
Clinton spoke to independent counsels Robert Fiske and Ken Starr a few times during his first term in office. Those weren't contentious interviews, and Clinton spoke voluntarily. But in 1998, once the matter of Monica Lewinsky's interactions with the President became part of Starr's investigation, Clinton's lawyers ran a more aggressive defense.
In the summer of 1998, Starr wanted Clinton to testify before a grand jury and he issued a subpoena on July 17, according to records from the Starr investigation. Clinton's personal attorney David Kendall tried to buy more than a week before he'd have to respond. In the last week of July, Kendall told Starr's team the President would be willing to testify.
Among Kendall's conditions: The President shouldn't be under a subpoena and he should testify from the White House with a time limit on the proceedings. Yet the two sides still couldn't agree on a date for Clinton's testimony, so they took their disagreement in late July before the chief judge in federal court in Washington. Before the judge could rule, Clinton's team agreed to have him testify on August 17, with a four-hour limit on the interview from the White House.
"There's no question that they could compel him to testify, unless he took the Fifth. The negotiation was all about the respect for the office," according to an attorney involved with the Clinton-Lewinsky investigation.
Previous court precedents worked against the Clinton team as well. There was US v. Nixon, another unanimous Supreme Court decision that ordered the White House to turn over the Watergate audiotapes after Nixon's lawyers claimed they should fall under executive privilege. Their claims for privilege weren't enough to overcome the needs of a criminal investigation, the court found.
There's also the case related to the investigation of Mike Espy, Clinton's first agriculture secretary, who resigned in 1994 amid an ethics scandal and independent investigation involving gifts. (A jury acquitted him in 1998.) His case attempted to use executive privilege to rebuff an independent criminal investigator's request for documents. The case reached a federal appellate panel of judges in 1997. The court disagreed with a lower-level judge's ruling that the White House could dodge an independent investigation's request.
The Espy decision also laid out a legal test that lawyers on both sides could use when a president faces an inquiry. A special counsel that subpoenas the president must seek evidence that's "important to the ongoing grand jury investigation and [must demonstrate] why this evidence is not available from another source."
The extent of Mueller's investigation or what he may want to ask of Trump is unknown at this point. An investigation focused on collusion with Russians during the presidential campaign may need Trump simply to serve as a witness -- a far more common perch from which past presidents have testified. However, the investigation could be focused on a possible obstruction of justice case related to Trump's decision to fire FBI Director James Comey. In that scenario, Trump could be a subject in the investigation or even its target.
If Trump is to testify, his lawyers and Mueller's prosecutors have a few options for how to tailor it. The lowest-stakes option would be for him to respond to written questions, called interrogatories. Trump's responses would be vetted by his legal team before they're sent to Mueller. Trump could voluntarily sit for an interview with Mueller's team -- and his lawyers could be present. That interview could either be under oath or not sworn in. Both approaches carry the same penalty if the person speaking to the authorities lies.
Trump could voluntarily agree to testify before the grand jury. If his team resisted, the prosecutors could compel the President to testify by using a subpoena, though it's likely Trump's lawyers would want to avoid that in the same way Clinton did. A subpoenaed appearance before the grand jury means Trump would walk in alone, to face just the jurors, the prosecutors and the judge. He would speak under oath.
In recent days, some of Trump's friends and advisers warned him he shouldn't talk to Mueller. Trump said, "We'll see what happens," when asked last week at a news conference about a possible interview with the special counsel.