Rex Tillerson gave a blistering assessment of his time serving as President Donald Trump's secretary of state in a rare public appearance, calling the commander-in-chief "undisciplined."

Tillerson told CBS News' Bob Schieffer at an event in Houston Wednesday night that he had never met Trump until the day he was asked to be secretary of state, and described how the pair were quickly at loggerheads once they began working together.

"So often, the president would say here's what I want to do and here's how I want to do it, and I would have to say to him, 'Mr. President I understand what you want to do but you can't do it that way. It violates the law,'" Tillerson said.

He added, "I'd say here's what we can do. We can go back to Congress and get this law changed. And if that's what you want to do, there's nothing wrong with that. I told him I'm ready to go up there and fight the fight, if that's what you want to do."

Tillerson, the former chairman and chief executive of Exxon Mobil, was fired as the country's top diplomat by Trump in a March tweet. He was replaced with then-CIA Director Mike Pompeo. Tillerson's firing came months after NBC News reported he called the president a "moron," which the oil executive publicly denied in a prepared speech.

Tillerson's Wednesday remarks are his first public comments since giving a commencement address in May at the Virginia Military Institute in which he deplored the nation's "growing crisis in ethics and integrity" and leaders who "conceal the truth."

Tillerson also told Schieffer that Trump acts on instinct and did not like to read intelligence briefings, which was challenging for him coming from corporate America.

"He acts on his instincts, in some respects, that looks like impulsiveness, but it’s not his intent to act on impulse. I think he really is trying to act on his instincts," Tillerson said.

“It was challenging for me, coming from the disciplined, highly process-oriented Exxon Mobil Corporation, to go to work for a man who is pretty undisciplined, doesn’t like to read, doesn’t read briefing reports, doesn’t like to get into the details of a lot of things, but rather just kind of says, 'Look, this is what I believe, and you can try to convince me otherwise, but most of the time you’re not going to do that.'"