"However, when it comes to whether or not the president should agree to a voluntary interview, the usual rules apply."

That's Jon Sale in his op-ed for The Hill.  Here's the intro to the piece:

For months now, pundits have propagated the myth that special counsel Robert Mueller and his team must interview President Trump to complete their investigation. This misconception is based on the belief that Mueller requires and is entitled to an interview to determine the president’s knowledge and intent. Mueller is not so entitled, and the president should not do it.

Government prosecutors conduct white-collar investigations every day. Usually, prosecutors complete their investigations without the benefit of interviewing the person under investigation.

Like all white-collar investigations, Mueller’s investigation requires an analysis of the president’s knowledge and intent. The allegations involving obstruction and foreign meddling in our election are no exception.

Knowledge and intent determinations are necessary in most white-collar investigations. Typically, prosecutors determine whether they can prove criminal intent based on the facts uncovered in their investigation, including relevant witness testimony and documentary evidence such as emails.

The prevailing view, based on legal opinions from the Justice Department’s Office of Legal Counsel, is that Mueller cannot seek an indictment of the president from the grand jury and, thus, his recourse is to write a report. That report can address the president’s knowledge and intent based on the evidence, without the benefit of a voluntary interview of the president.

While it is “typical” for a prosecutor to conclude an investigation without interviewing the person under investigation, this investigation is anything but typical. However, when it comes to whether or not the president should agree to a voluntary interview, the usual rules apply. The president’s legal team is wise in recommending that he not agree to an interview; all it could do is help Mueller’s team advance an arguable case against the president.