If John Bolton was expecting an easy interview with Stephen Colbert, he was way off base.

Appearing on The Late Showon Tuesday to promote his tell-all book, The Room Where It Happened, Donald Trump's former National Security Advisor found himself fielding some fairly tough questions from the late night show host.

Questions like why, when Bolton knew what he knew about Trump's personality, did he go to work for the president in the first place?

"I guess what's exasperating is there is absolutely nothing Donald Trump has done that is surprising to me," says Colbert. "My rule is, everything you thinkabout Donald Trump is probably true, because he's not deep enough to get your socks wet in. He's incredibly readable — that's why when he ran casinos, the house lost. There's nothing to learn about him. That's why he's essentially a boring person. How did you not know beforehand that he was just callow?"

Bolton's response? "Because I couldn't believe it was that bad."

"But you're an international negotiator," replies Colbert. "How could you be naive? You've dealt with the worst people in the world. "

There are four parts to the interview available on YouTube, each one as fiery as the next. For example, in the video below, Colbert asks Bolton about his criticised decision not to testify in Trump's impeachment trial, as someone with firsthand information that could have impacted the president's defense. Bolton defends his position by saying that the impeachment effort was already "in a ditch" due to "the way the Democrats had structured it, they were looking for a partisan fight," and he therefore thought his testimony "would not have made any difference."

"Because it was prosecuted poorly in your opinion and too narrowly, isn't that like someone saying, 'I happen to know that this guy murdered a lot of people, but because they're not prosecuting him as a serial killer, I don't want to testify in this one murder that they found?'" asks Colbert.

Bolton argues that it wasn't just, as he refers to it, the scope of the impeachment, but also "the way the House Democrats treated House Republicans" during the process. "I believe that there were some substantial number who would have been open to a fair consideration of a range of issues," he says. "That was never part of the Democratic plan. So that many Republicans who were no fans of Donald Trump ended up in the position of having to defend him."

Colbert doesn't let that one slide, though.

"Sir, I understand your argument, but you've already said it was a fair case against him and an impeachable offence," says Colbert. "All you're saying is today's Republican party, far from not being invited into a bipartisan process, are merely so toxic and so partisan that even though they know he is guilty, they will not do what's best for the American people."

Watch the whole interview in four videos available on YouTube.