Guardian: Reports " Federal prosecutors often treat them as fishing expeditions. Witnesses can be questioned under oath for hours on end, with investigators hoping to trip them up and make them perjure themselves. "
Lets Get Real:
This is the kind legal process that can catch those that think they are more clever than the lawyers, in this case it would be POTUS. As the article notes a President can be forced to give evidence to a Grand Jury, one can see POTUS thinking he knows more than the lawyer and the general public of the Grand Jury and getting caught out by the special prosecutor. If Trumps is going to sack the SP he should do it now, otherwise we are in to Watergate territory. Thus years of Congressional and Legal cases, the US voter wants his/her problems solved not another D.C crisis.