During her time as Home Secretary, she infuriated officials at times with her agonisingly slow decision-making. Once the facts were assembled, however, they learnt she was more than prepared to make brave moves, off-piste responses that might have been unexpected but were never ill-considered.
Before the Election that Will Be (in contrast to Gordon Brown's 2007 Election that Never Was), perhaps the quintessential example of May's capacity to electrify Westminster with cautious boldness was her 2012 order to block the extradition to the US of the computer hacker Gary McKinnon.
Her civil servants had asked for her decision within weeks of her appointment as Home Secretary. She took two-and-a-half-years.
Defying the Obama Administration upended the norms of the 'special relationship' - a wildly unorthodox move that stunned the then Prime Minister, David Cameron, and his Cabinet, whom she spectacularly failed to consult.
It also proved overwhelmingly popular with the British public and, once made, seemed utterly logical, for all its unexpectedness.
The modus operandi that allowed May to survive the graveyard of political careers that is the Home Office has now been transplanted to Number 10.
As with the McKinnon decision, she will have discussed the prospect of an early election with no one bar a tiny coterie of trusted advisers, including her two consiglieri, Fiona Hill and Nick Timothy, and her husband, Philip, whose down-to-earth advice she prizes above all others'.
So tight is this inner circle that not a whisper of the Prime Minister's change of heart over the election escaped Number 10 before her big announcement, to the extent that the most seasoned Westminster watchers were unsure what she would say until she opened her mouth.
What a contrast to Brown, another Prime Minister with a reputation for caution, who, when confronted with a similar choice of whether to take advantage of favourable polls to go to the country early, allowed rampant speculation among his loose-lipped advisers.
Where May has displayed considered boldness, Brown's decision not to call an election nearly a decade ago was an example of dithering timidity from which his party has never recovered.
It is, above all, the shattered state of the Labour Party under Brown's successor-but-one, and polls that indicate a thumping Tory majority, which have made up May's mind for her.
The overwhelming logic of calling the election now, when all the stars are aligned, rather than waiting three more years, two of which will be consumed by Brexit, is such that it begs the question why it took her so long to come around to the idea.
For the answer, we must examine her biography. As a child of the vicarage, it sticks in the craw for May to break a promise, not so much to her media interrogators as the electorate at large. Having pledged at the outset of her premiership to reign until 2020, she was minded to keep her word.
But with the polls last weekend so favourable they might have put mettle into Brown were he in her - leopard-print - shoes, the temptation to pursue a majority most prime ministers could only dream of was too much, even for the vicar's daughter.
- Rosa Prince is the author of Theresa May: The Enigmatic Prime Minister
