I really have to wonder, though, exactly how firm that policy really is. Any crime? Say the sitting president shot someone in the middle of 5th Avenue, on carmera, in front of a huge number of witnesses. No charges until after impeachment and removal from office?
No, it’s no charges while in office for things that are related to presidential duties, but that can be stretched as it’s very vague.
No, we’re referring to the 1973 Office of Legal Counsel Memorandum:
In 1973, amid the Watergate scandal, the Department of Justice’s Office of Legal Counsel (OLC) issued a memorandum concluding that it is unconstitutional to prosecute a sitting president.[22] Its arguments include that the president “is the symbolic head of the Nation. To wound him by a criminal proceeding is to hamstring the operation of the whole governmental apparatus in both foreign and domestic affairs.”[23] It says that the statute of limitations should not be tolled while the president is in office, but suggests that Congress could extend the statute of limitations specifically for presidents.[24] After the U.S. Supreme Court’s decision in Clinton, the OLC issued a second memorandum in 2000, distinguishing civil and criminal presidential immunity and determining that it was still improper to prosecute a president due to the adverse affect it might have on his ability to govern.[25]
Like you said, it’s a memorandum. If someone committed, like homicide and cannibalism, a prosecutor would probably file charges and then it’d be up to a judge to determine if the reasoning in the memorandum is correct.