Playful refusal to bare all keeps us guessing

It is no coincidence that Sir Alec Guinness, the most private of great 20th-century British actors, eschewed the showy classical stage roles and found international fame by playing a series of subtle, almost anonymous, characters on film. Guinness, the actor, refused to take centre stage and reveal all, and Guinness, the man, shied even further away from self-revelation.

It is rare for us to know so little about a public figure who died as recently as 2000, but even from beyond the grave Guinness's privacy remains intact. His papers have not been made public, his published words are characterised by a playful ambiguity and even his closest friends are not sure that they ever really knew the man. This is the problem facing Garry O'Connor in his unauthorised, but in many ways definitive, biography.

It is hard to get a hold on such an elusive subject. One story illustrates this. O'Connor is keen, almost to the point of obsession, to prove that Guinness, despite his long and devoted marriage to his apparently saint-like wife Merula, was homosexually inclined. O'Connor's interviewees - an impressive array of Guinness's friends and colleagues - concur, but provide little proof. No lovers are interviewed, or named. The most widely known story concerning Guinness's sexuality involves his arrest for homosexual soliciting in Liverpool in the 1940s. Unlike Sir John Gielgud, who was caught out in a similar fashion ten years later, Guinness had the foresight to furnish the police with a false name: Herbert Pocket, the character in the film adaptation of Great Expectations that had just made him famous. The story is repeated here, but O'Connor can find no proof that it occurred - there is no mention of the case in public records and arrests for homosexual soliciting in Liverpool at that time were few and far between.

O'Connor turns this elusiveness to his advantage, piecing together the facts and combing interviewees' statements with forensic rigour, before making an imaginative leap into a compelling psychological portrait. He subtitles his book "The Unknown" but he succeeds in bringing Guinness vividly to life without diminishing the mystery surrounding the man.

Spirituality is a hard notion to pin down and Guinness' reasons for converting to Catholicism in middle age remain vague. O'Connor believes that he found refuge in faith from his own demons, but we will never know for sure. What we do know is that as he grew older Guinness began to play, almost exclusively, spiritual characters of some kind, prompting the criticism that he always seemed to be playing the pope.

O'Connor makes the connection between Guinness' inner life and his acting roles persuasively. He argues that Guinness was driven by the shame he felt at his illegitimacy and his homosexuality, and that fear of being found out conditioned the kind of actor that he became: an artist working in a minor key. Guinness employed small gesture and subtle expression to suggest, or as Time magazine put it: "His essential gift is not for creating characters, but existences." Guinness, as an actor and as a man, was complex and guarded, but liked coyly to give the merest suggestions of what lay submerged, as though daring exposure. In the end, Guinness comes across as a mildly flirtatious maiden aunt performing a strip-tease but unwilling to bare all - a glimpse of stocking here, a flash of thigh there. And it is the parts that Guinness kept well covered that formed the wellspring of his genius.

Robin Dashwood is a television drama and documentary director at the BBC.