Former secretary of state Hillary Clinton set both as tests of grit and leadership. Email practice contributed to her failed 2016 presidential bid; and in the heat of her failed run for the Democratic presidential nomination in 2008, she ran TV ads claiming she was better equipped than her rival Barack Obama to deal with crises that would erupt in the middle of the night.

Happily, Rex Tillerson, former ExxonMobil CEO and Donald Trump's Secretary of State, passed his early-hours test - it happened in impoverished Yemen. But the jury is out on his emails. There is a tantalising probe into the extent to which ExxonMobil misled shareholders on the impact of their products on climate change that bubbles along in New York and which may, or may not, prove embarrassing for Tillerson.

The Yemen business was a try-on by a Yemeni prime minister who, after three days of haggling in Sanaa, the Yemeni capital, had struck a big-ticket energy deal with the young Tillerson - or so Tillerson believed.

An exhausted Tillerson had crashed, sleeping ahead of a 6am flight home to Texas. But his phone rang at close to 3am - it was the PM, demanding an immediate reopening of the negotiations. Tillerson hauled himself back to the government bunker - the Yemeni played hardball, Tillerson didn't blink.

"The Prime Minister tried to retrade the deal and Rex just basically just said 'No,'" Politico magazine was told. And this, a Tillerson associate explained, was proof that "Rex can say 'No', and do so in a way that is not offensive and doesn't create unnecessary battles - the government of Yemen wasn't going away and we needed to deal with it."

The gist of the email business is this - ExxonMobil is accused of replicating the tactics of big tobacco, funding climate change deniers, despite knowing its energy products contributed to climate change, just as the tobacco giants covered up that their products were killing their customers.

Responding to a demand for documents by New York State Attorney-General Eric Schneiderman, ExxonMobil reluctantly revealed the existence of a trove of Tillerson's emails, sent as CEO but under the alias "Wayne Tracker", which had been "lost" for reasons the company insists are innocent. Schneiderman is plugging away and ultimately ExxonMobil and Tillerson may be cleared, but for now the optics are a messy reminder of Clinton's obsessive and ill-judged need for secrecy in the saga of her emails.

Tillerson's transition from the corporate sphere to the world of diplomacy - and the diplomacy of the world - has Washington transfixed.

It's early days and he does have strong defenders. But in switching from being his own master to serving Donald Trump, from being answerable to 2.5 million narrowly focused shareholders to being accountable to more than 300 million citizens of all views and opinions, Tillerson is widely criticised for failing to project himself and his issues; of being cowed by a bullyboy President and his inner circle; and of failing to grasp, or to be even mildly interested in, the survival essentials of bureaucratic and political life.

The new Secretary of State is cast as the hermit of Foggy Bottom, a cognomen the State Department takes from the DC quarter in which it is located. Some of Tillerson's senior management walked out even before his Senate confirmation and on arriving he showed more the door. The White House refuses to allow him to make his own senior appointments, and, something of a sacrilege in Washington, he shuns the media.

"He's isolated and he's not using the leverage of his office," Ilan Goldenberg, of the Centre for a New American Security, told Fairfax Media. "He's fundamentally disconnected - people at the State Department don't know what's happening on the seventh floor."

If Tillerson is strangely absent from the brightly lit Washington stage, he is mentioned only obliquely in wonkish assessments of the Trump national security team, which, generally, focus on the role in government of Defence Secretary Jim Mattis, of National Security Adviser General H. R. McMaster and of Homeland Security Secretary John Kelly.

As with other Washington portfolios, the self-styled "wrecking ball" Trump is disruptive when it comes to foreign policy. He'll happily take out his angst with the world on Twitter, thereby requiring his cabinet appointees to wind back positions more in keeping with the challenging credo of the Bannon-Miller axis, than they are with the more conservative, establishment thinking of the likes of Mattis, McMasters and Tillerson.

In January, Team Trump hailed Tillerson's nomination, claiming it would "help reverse years of misguided foreign policies and actions that [had] weakened" the standing of the US in the world. But the new man is no carbon copy of his leader and neither is he as two-dimensional as depicted in some media profiles.

While Trump was leaning towards lifting sanctions on Moscow, imposed after his annexation of Crimea, Tillerson told his Senate confirmation hearing that the West should be alarmed by Russian aggression; unlike Trump, he voiced strong support for the US commitment to NATO and he endorsed US arms for Ukraine, to help it to repel Russia's push into its territory.

In Beijing earlier this month, Tillerson displayed more realpolitik than Trump belligerence in his dealing with Chinese officials - to the point of using Chinese phraseology, which was read in the Chinese capital as his endorsement of the Chinese belief that Washington should embrace China's stand on the South China and East China Seas and Taiwan and on human rights in Tibet and Xinjiang.

ExxonMobil is defending its record on climate change. But Tillerson can claim that it was on his watch as CEO that the energy behemoth embraced scientific arguments on greenhouse gas emissions and climate change - which anchors him more in scientific reality than Trump.

And philosophically, he's a global free trader - again putting him at odds with the President. In a conference speech in 2007 he declared: "Energy made in America is not as important as energy simply made wherever it is most economic."

As national president of the Boy Scouts in the US, it was through Tillerson's advocacy that the movement agreed to accept gay members. And he is a former director of the United Negro College Fund, a group that funds scholarships for black students and supports historically black colleges and universities in the US.

Still, Steve Coll sees great irony in Tillerson's appointment. The author of Private Empire: ExxonMobil and American Power, Coll writes in The New Yorker: "Some of the Exxon executives I interviewed spoke about Washington with disdain, if not contempt. They regarded the State Department as generally unhelpful, a bureaucracy of liberal career diplomats who were biased against oil and incompetent when it came to sensitive and complex oil-deal negotiations. They managed Congress defensively, and as just one capital among many in the world, a place more likely to produce trouble for Exxon than benefits.

"In nominating Tillerson, Trump is handing the State Department to a man who has worked his whole life running a parallel quasi-state, for the benefit of shareholders, fashioning relationships with foreign leaders that may or may not conform to the interests of the United States government."

Tillerson is a unique appointment. His entire career has been in the service of ExxonMobil and he takes office having done neither government nor military service. But he knows his way around the world and he knows how to cut a deal - though the manner of some have raised eyebrows.

Trump did not go into specifics on the appointment, but some analysts have picked on the President's narrow focus on the intersection of foreign policy and US economic interest - as borne out in Trump's "take the oil" claim on Iraq and his short-lived willingness to use Taiwan as a bargaining chit with Beijing.

On one level, Tillerson is a novice. On another he's not. In running ExxonMobil, he presided over a global staff of 75,000 with operations in as many as 200 countries and constantly having to finesse deals with the likes of Vladimir Putin in Russia; Hugo Chavez in Venezuela, Idriss Deby in Chad and Ali Abdullah Saleh in Yemen.

"Those early assignments where I worked internationally and the things I learned there have served me well ever since," Tillerson told an interviewer in 2007.

It remains to be seen how he'll get along with Baghdad, given a 2011 oil development deal he cut with the Iraqi Kurds - he left the central Iraqi government out and undercut Washington's support for a national deal under which Kurds, Shiites and Sunnis would benefit from the country's vast oil reserves.

He became very close to Putin who awarded him Order of Friendship in 2013.

Some analysts are troubled by Tillerson's settling-in at the State Department. They see no push-back against a White House takeover - Trump wants to cut Tillerson's budget by about 30 per cent; Trump, his son-in-law Jared Kushner and strategist Stephen Bannon are seen to be running a parallel "State Department" from the White House; Tillerson is locked out on the appointment of envoys; and Trump has twice embarrassed Tillerson while he has been abroad - blasting China via Twitter for not doing enough to ease the North Korea crisis while Tillerson was in Beijing; and crowing about how he'd be getting rid of "bad dudes" from the US while Tillerson was in Mexico City.

Inside-the-beltway conspiracy theorists speculate that Bannon is calculatedly attempting to sideline the State Department, citing as proof the very public warning by now sacked National Security Adviser Mike Flynn that Iran was "on notice" not to attempt further missile tests, and the quashing of Tillerson's move to appoint former Bush official Elliott Abrams as his deputy.

Tufts University's Professor Daniel Drezner is scathing. He told Fairfax Media: " It's possible [Tillerson] will learn the lie of the land and be competent, but there is zero evidence. Beyond Trump's [belated] adherence to the One China policy, I've seen nothing [Tillerson] has done that is constructive ... and he seems to be willfully alienating the State Department."

Henry Kissinger, the grandee of Republican foreign policy, was diplomatic after meeting Tillerson. "The normal tendency when you come into that job is to increase your visibility and to show that you are present and in charge," he told The New York Times. "[But] he wanted to first inform himself of all the nuances. I was impressed by the confidence and self-assurance that he showed."

Tillerson is seldom seen and not often heard - he shuns public appearances; is absent from media coverage of Trump's meetings with foreign leaders; and is seemingly resigned to the White House making the final decision on who is to work at State and to Trump's plans to hack into his budget.

Theories on his low-key arrival are many - he wants to lock in key administration relationships before committing himself on policy; he's waiting for the dust to settle in the factional wars at the White House; and that at the end of the day, he's an energy CEO - you cut a deal, you go home and you say as little as possible.

Some foreign diplomats and former officials reject the intensity of media criticism of Tillerson, and particularly of his reluctance to have the press travel with him.

Former senior Obama appointee and Asia policy expert Evan Medeiros told Politico: "that's Washington navel-gazing. That's not what the region is preoccupied with ... [The Chinese in particular] like to deal with people that are cool, careful, discreet ??? but very credible interlocutors. And I think right now they see Tillerson like that."

Explaining himself in a rare interview, Tillerson said: "I'm not a big media press access person. I personally don't need it ... When I have something important and useful to say, I know where everybody is and I know how to go out there and say it.

"But if I don't because we're still formulating and we're still deciding what we're going to do, there is not going to be a lot to say."

Suzanne Maloney, a former ExxonMobil staffer now at the Brookings Institution, rushed to Tillerson's defence on Twitter: "Oil folks know stuff: anyone who manages multibillion-dollar, multi-decade projects needs deep, nuanced understanding of political context ??? Tillerson rose to top of a company that prizes technical excellence, rock-solid financials, hard work and integrity. State could do a lot worse."

But a worry, as expressed by Thomas Wright of the Brookings Institution, is though Trump may be getting boxed in on much of the foreign and national security agenda by the establishment sensibilities of Mattis, McMasters and Tillerson, at age 70 this President is not likely to change, because he feels vindicated by winning the election "after a lifetime of howling at the moon".

So how will Trump respond when, inevitably, he gets the 3am call? Will his national security team get time to reel him in, or might they have conditioned him in time to be as measured as Americans need him to be when the balloon goes?

What will be Tillerson's role in all that? Trump picked this Secretary of State on the recommendation of former defence secretary and CIA director Robert Gates - a Bush appointee kept on by Obama.

This is how Gates celebrated Tillerson's credentials in January, during the latter's Senate confirmation hearing: "Against a backdrop of having known, or worked with 12 secretaries of state, I believe Mr Tillerson is superbly qualified ... He is deeply knowledgeable about the international scene and geopolitics. And importantly, would be an informed and independent adviser to the President."