President Trump has more than a few characteristics that make him a challenging boss. He is mercurial, quick to turn on subordinates deemed insufficiently loyal, and has a slippery relationship with the truth.

Those traits help explain why so many of the President’s associates in business and government felt compelled to document their interactions with him.

Out of self-preservation, a desire to keep an accurate record, a lack of trust—or some combination of the three—everyone from former FBI Director James Comey to former Trump “pitbull” Michael Cohen created records of their encounters with Trump. These contemporaneous notes, official memos, and audio recordings will be invaluable evidence for prosecutors investigating whether Trump obstructed justice or engaged in other possible misconduct, former federal prosecutors tell TPM.

“If you have a recording or contemporaneous documentation of something that happened, it’s like a golden egg,” Ashlee McFarlane, former assistant chief in the fraud section of DOJ’s Criminal Division, told TPM.

“Oftentimes it’s more accurate than a later recalling of an event,” McFarlane added. “The fact that the person was prompted to record it shows a mindset of how they felt about the encounter. And people are less likely to fabricate something at the time, in the moment. They have less incentive to do that.”

Trump’s own stream-of-consciousness Twitter feed is reportedly one source for Special Counsel Robert Mueller on the question of obstruction. But external documentation from credible sources creates a more complete picture.

One fruitful example: the memos Comey made recounting every one-on-one interaction he had with Trump. Thanks to those, we know about Trump’s Jan. 27, 2017 request for Comey’s “loyalty,” his Feb. 14, 2017 request that Comey let the FBI’s investigation into ousted national security adviser Michael Flynn “go,” and his March 30 plea that Comey help “lift the cloud” caused by the investigation into Russia’s election interference.

Comey testified before Congress last year that he “felt compelled” to create these detailed, contemporaneous accounts of their conversations immediately after they occurred and share them with senior FBI leadership. This compulsion did not strike after his two private conversations with President Barack Obama during the prior administration, Comey said.

The difference, according to Comey, was “the nature of the person.”

“I was honestly concerned that he might lie about the nature of our meeting, and so I thought it really important to document,” Comey told the Senate Intelligence Committee.

Trump’s repeated requests to speak with Comey alone made the FBI director “uneasy” and stirred a “gut feeling” that prompted him to document their encounters.

“The circumstances that I was alone, the subject matter, and the nature of the person that I was interacting with and my read of that person” provoked him to take these steps, Comey testified.

A similar sense of unease reportedly prompted Comey’s successor, former acting FBI Director Andrew McCabe, to create a confidential memo detailing his account of Comey’s abrupt firing.

The special counsel is investigating the cause of that dismissal as part of his probe into Trump’s possible obstruction of justice. Trump initially pinned the firing on a pair of memos written by Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein criticizing Comey over his handling of the Hillary Clinton email investigation.

The May 2017 McCabe memo complicates that narrative. As the New York Times reported, McCabe’s document recalls a conversation he had with Rosenstein shortly after Comey’s firing. In that conversation, according to the Times, Rosenstein said that the president has originally asked him to reference the Russia investigation in his own memo. McCabe interpreted the remark as possible evidence that Comey was actually fired because of the Russia probe—a suspicion confirmed by Trump himself, who told NBC News that “this Russia thing” is what ultimately forced his hand.

“It’s almost like they anticipated being witnesses and wanted to make sure they preserved their recollections,” McFarlane told TPM of Comey and McCabe.

Creating memos of any “meaningful, substantive conversation” is also just standard practice at the Justice Department and FBI, former federal prosecutor Elie Honig told TPM.

“There’s a deep culture in DOJ of recording anything that’s significant and keeping those notes,” Honig said, which can be used to refresh witnesses’ memories, keep dates in order, and push back on inaccurate claims by defense lawyers.

It wasn’t just trained intelligence professionals who created records that could create trouble for the President. Trump’s own White House counsel reportedly did so too.

The New York Review of Books reported this week that the special counsel is in possession of a Feb. 15, 2017 memo created by White House Counsel Don McGahn outlining the set of events that led up to Flynn’s firing. That timeline “explicitly states that when Trump pressured Comey he had just been told by two of his top aides — his then chief of staff Reince Priebus and his White House counsel Don McGahn — that Flynn was under criminal investigation,” according to the report.

This document, compiled by McGahn and two of his deputies, contradicts Trump’s personal attorneys’ explanation for his Feb. 14 request that Comey drop the Flynn probe. They have said that the president could not have obstructed justice because it was the “White House’s understanding” that there was no active FBI investigation into Flynn.

Federal prosecutors also have a trove of recordings from a valuable source outside of government: Michael Cohen. Once one of Trump’s most loyal enforcers, Cohen has turned on his boss, hinting publicly that he’s willing to cooperate with the U.S. Attorney’s Office in Manhattan’s criminal investigation into Cohen’s business dealings—even if that means ratting on Trump.

In a recently-released recording of a fall 2016 in-person meeting, Cohen tells Trump he spoke with Trump Organization CFO Allen Weisselberg about handling a payment involving a former Playboy model who said she had an affair with Trump.

Cohen had a general habit of recording in-person and phone conversations as a way of taking notes, his lawyer Lanny Davis has said. But Trump was apparently unfamiliar with this practice, lashing out at his former lawyer for the “possibly illegal” recording of a client without their knowledge.

Weisselberg has reportedly been subpoenaed to provide testimony before a federal grand jury in New York, providing prosecutors with an opportunity to question him about the Trump Organization’s finances.

Trump is known for being something of a luddite, even explicitly saying at a 2016 campaign rally that he eschewed email to shield himself during legal disputes.

If only the President could say the same about those around him.