It may sound surprising for someone who has just won the Whitbread biography award for a life of Mary Queen of Scots, but I never intended to write a book about her. My fascination always lay with Elizabeth, her cousin and rival for the English throne. I wanted to examine a woman ruler’s dealings with her male advisers, to see how the male-dominated system coped. Only later did I see Mary and Scotland as the heart and soul of the story.

Working from the original documents, the deeper I dug the more I realised William Cecil, Elizabeth’s chief minister for almost 40 years, spent more time worrying about Mary than about his own queen.

Mary was Cecil’s obsession and religion was the reason. Mary had the best claim to the English throne if you were a Catholic, Elizabeth if you were a Protestant. Cecil was a messianic Protestant. He agreed with John Knox, the doyen of the Scottish Calvinists, that a Catholic woman monarch was “a monster in nature” and “unfit to rule”.

Both men yearned for a united Protestant Britain, fearing Mary less for what she did than for her mere existence. Her affability compounded their disdain. Mary was intelligent, vivacious, kind, loyal to her supporters and friends, and devoted to her family. She used charm as others used a sword.

Far from matching Knox’s stereotype, she was successful for much of her reign, relishing power and holding together a country deeply divided over religion. She could be just as shrewd and masterful as her English cousin, whose misjudgments are too readily glossed over.

Mary’s failings were reassuringly human. She was too generous to her enemies, too trusting of her friends. Her worst mistake was to allow herself, a queen, to fall in love.

How was I to get inside Mary’s head? First, I tried to root everything I said about her in the original documents, notably her hundreds of letters. Next, I’d try my arguments on women historians, including my girlfriend Julia Fox who endured many a late-night discussion on how Mary must have felt and thought.

At first I believed Elizabeth was successful where Mary failed. Now I think the gregarious, accessible Mary, with her courage, verve, wit and appreciation of life, is far more compelling. Mary wanted to heal the wounds of civil and religious strife in Scotland: she had a policy of national reconciliation. Elizabeth’s aims were narrower. For all her vaunted rhetoric, she was remote, caring little for her people. Tougher, grittier, more impervious to the ties of kinship or friendship, she succeeded because she lacked ideals.

When my book was first reviewed, some critics insinuated that I’d fallen in love with my subject. While I freely concede that I set out to tell Mary’s story, that doesn’t mean I accept her version of events uncritically. I tried to be as objective as I could. I quoted her extensively because it isn’t just what she says, but the way she says it that defines her character. Her choice of language reflects her values.

I was also amazed to discover resonances of Blairite Britain in my documents. When Mary fled to England, she was denounced by Cecil as a threat to national security. He had her imprisoned without legal recourse as a viper at the heart of an international conspiracy. He talked relentlessly of Elizabeth’s “safety” and “the preservation of the state”.

When, despite all his efforts, Elizabeth took Mary’s side, proposing to restore her as queen in Scotland, insisting that the principles of monarchy overrode those of Protestantism, Cecil turned to spin. He colluded with Mary’s rebels, whom he encouraged to send a sexed-up dossier to London, then organised a tribunal of inquiry that he intended to fix. Yet Elizabeth, unconvinced by spin, insisted that the inquiry and the dossier be kept secret.

Cecil finally got his way and Mary was executed, but Elizabeth made him wait 19 years. Even then, she countenanced Mary’s death only after Cecil had falsely reported that the Spanish Armada had landed in Wales. Elizabeth was put in fear of her life, and by then Mary was desperate for freedom and involved in a genuine conspiracy to win it.

Mary’s story is so theatrical, it’s no surprise that Hollywood hopes to capitalise on it. Warner has said it will make a film, written by Jimmy McGovern, of Cracker fame, with the American actress Bryce Dallas Howard playing Mary. It sounds like a dream team, but Mary’s story is fiendishly complicated and audiences will need to keep their historical wits about them.

The film rights to my book have been sold to David Rose, a British independent film producer willing to mount a David and Goliath fight with Warner and combine good drama with good history.

Whatever happens, you’ll be hearing an awful lot more about Mary.

My Heart is My Own: The Life of Mary Queen of Scots by John Guy is published by HarperCollins at £8.99

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