His face was stretched taut with nerves. His top lip appeared to be locked solid. As the Iraq inquiry's chairman, Sir John Chilcot, told the world that this was not a trial, the witness's hands opened a bottle of water, his hands visibly shaking.

Tony Blair clasped both hands together in front of him to steady himself as Sir John expressed the hope that the inquiry could go about its business in an orderly way without disruption. A burly security guard sat in the room just in case. The former prime minister stared straight ahead, barely blinking. I have not seen him so frightened since the evening I welcomed him backstage to take part in a live televised debate between the contenders for the Labour leadership 16 years ago.

As on that night, though, the nerves didn't last long.

The man who for so long relied on his capacity to charm and to persuade had, it seemed, decided that he must not be seen to do that. Thus, there was no opening statement or preamble of remorse for those who died in Iraq; nor were there thanks for the opportunity to give evidence.

Instead, he and Sir Roderick Lyne, the best inquisitor on the team, sized each other up. Blair repeatedly put on then took off his glasses as he reached out for the speeches stored in front of him in a lever-arch file, unsure whether to read them out. "We'll come to that," said Sir Roderick, to demonstrate who was in charge.

Soon, though, the witness was at ease, his face relaxed, his eyes more lively. As he warmed to his own tune, his hands began to move expansively - as though he were a conductor who had at last found the beat and was beginning to enjoy it.

Before Mr Blair had entered, the audience - in part invited relatives of soldiers who had died; in part members of the public who won their places in a ballot - had sat in quiet contemplation. Many had arms folded, looking as tense as the man about to appear before them. Only once did those of us in the room hear a reaction not audible to those watching on TV.

It came when Mr Blair was asked about the interview with Fern Britton, in which he appeared to say that if he had known before the war that Saddam had not possessed weapons of mass destruction, different arguments would have had to have been used to justify removing him. The Blair of old grinned, and then joked that, even with all his experience of doing interviews, he still had things to learn.

There were sharp intakes of breath, there were audible tut-tuts and there was shaking of heads - a low-key but collective expression of resistance by an audience who appeared to say: "don't think you can get away with that one."

It was on that issue - regime change - that we learned the most this morning.

Up until today, witnesses from Tony Blair's government have insisted that the Americans' stated objective of regime change was illegitimate and illegal. The British government's policy of disarmament was distinct, they insisted. However, Tony Blair said this morning that there was no "binary" choice between them and that they were, indeed, different ways of expressing the same proposition.

One particular phrase sticks in my mind. Even before the attacks on New York, he told the inquiry, "force was always an option... if necessary, we were going to remove him."

Remove him. Regime change. It had always been in his mind. Long before that meeting at George Bush's Crawford ranch where some allege he made a promise "signed in blood" to go to war.

PS: I will be back inside the inquiry room for the last session of evidence this afternoon. My colleague Laura Kuenssberg is micro-blogging; you can find her at @BBCLauraK, and you can get all the BBC's coverage at our live event page.