This book grew out of the research Simon Callow did for a play, Inside Wagner’s Head, which he wrote for the composer’s bicentenary in 2012. What was it about this man, he asked himself, that made him so controversial – in his day and since? It is an actor’s book and he came up with an actor’s answer: his subject’s “demiurgic personality”. Wagner was a man without boundaries, a “man connected to his ‘inner infant’”; a master dramatist of his life and an actor of astounding gifts (“more an actor than a composer,” Nietzsche thought); a man who, for better or worse, imposed himself and his work on the world as perhaps no other artist in history had done.

Callow was on the right track in looking for some almost preternatural capacity for self-creation as an explanation for why the question of Wagner still exercises people. It is not primarily because of his views, notably his notorious antisemitism. There were lots of radical antisemites in the 19th century – Count Gobineau, or Hitler’s favourite, the Austrian Georg von Schönerer, for example; there were other composers – Mussorgsky and Liszt – who had unkind things to say about Jews. Today they excite very little controversy outside footnotes. Wagner was controversial before Hitler came to imagine himself as Rienzi, the hero of the composer’s first successfully staged opera, and before Siegfried’s funeral music from Götterdämmerung came to be associated with Nazi mourning. (It wasn’t just the Nazis who appropriated this music for ritual purposes. A brass band of 500 played it to accompany a cannon salute at Lenin’s funeral. A real orchestra played it again at the Bolshoi.)

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Nor is Wagner controversial because he was such a difficult character. Lots of artists are that: they are in general “driven by demons”, “completely amoral”, thought William Faulkner. “A man must be a very great genius to make up for being such a loathsome human being,” one of Ernest Hemingway’s wives said about him. But no one is really interested in running a balance sheet in the case of Wagner: the debit side would be crushing. Wagner is more than bad; he is pathologically bad. His “music grows parasitically”, writes the composer Thomas Adès whom Callow quotes: “It has a laboratory atmosphere – a sort of fungus.” “Is Wagner actually a man?” Nietzsche asked. “Is he not rather a disease?” At stake here is far more than being a horrible person, a fact that Wagner did little to hide.

His radically innovative, embarrassingly voluptuous, riveting – or, some will say, boring – music is at the heart of the controversy, and of Callow’s attraction to his subject. He says he has been a Wagnerian since early adolescence: he knew all about leitmotiven and the “Tristan chord”. But pointing to his music is only to push the question one step back: why? Lots of composers before Wagner used the same notes in a chord but he managed to keep it unresolved from the beginning of a work to orgasmic ending – five hours of Tantric harmonic deferral. That got the naming rights, one supposes.

Beethoven, the only composer who comes close to Wagner in his daring breaks from the past and who was met initially with a similarly uncomprehending and hostile reaction, was well on his way to being the assimilated prototypical genius of the 19th century within 20 years of his death. In fact, Wagner’s Dresden performances of Beethoven’s Ninth in the 1840s played an important role in inserting a wild and unruly symphony into the heart of the musical canon. Wagner’s operas are unquestionably canonical but they still generate the sort of hostility they did when they were first performed. None of the great 20th-century masters – not Stravinsky, not Schoenberg, not Boulez – is as divisive today as this composer born more than 200 years ago.

In this book, as in the 2012 play, Callow is still engaged with what was going on inside Wagner’s head – “What was it like to be Richard Wagner?” But he expands on that question here: “What was it like to be with Richard Wagner?” And, more revealing, “What was it like to become Richard Wagner?” It is a book about the production of a man for whom “self-dramatisation was his essential mode” and who, in his autobiography My Life, set the standard.

For some, Wagner was bewitching, hypnotic; for others he was unnerving, dangerous, overwhelming

Actors need to develop an intuitive understanding of what it would be like to be the character they are playing. Few can be harder to figure out than Wagner; few are more shrouded in the theatricality and contradictions of self-fashioning. Here was a man who was for much of his life an outsider – 15 years in exile for his role in revolutionary hooliganism in 1848. Constantly fleeing creditors and those he had wronged, he set himself the task of German spiritual regeneration. He hated capitalism and the cash nexus but was also endlessly asking for money. Some of his begging letters and responses to those who failed to meet his demands are masterpieces of penurious self-important mendacity: “I hear that you have become rich,” he writes to Robert von Hornstein. “I require an immediate loan of ten thousand francs.” He recognises that this will be hard for his would-be patron, but reminds him that it is possible if he really wishes it and that a sacrifice for Wagner is worth it. When Hornstein turned him down, Wagner dashed off a letter saying that the opportunity to help a man like him is unlikely to happen again. Few have lived their lives with so unguarded an unconscious.

It is easier for Callow to get at what it was like to be with Wagner. No one who knew him was left untouched; many wrote about the experience. For some he was “bewitching, positively hypnotic”; for others, including those close to him, he was “unnerving, dangerous, overwhelming, almost life-threatening”. Wagner could not keep quiet. He “had an enormous gift of the gab”, Robert Schumann noted in his diary, but was full of oppressive ideas and “impossible to listen to for any length of time”. Nietzsche wrestled with his sense of betrayal by his one-time friend and master whom he had serv, often in humiliatingly abject waysed for years: “One pays heavily for being Wagner’s disciple.” The breakup meant little to the composer; Nietzsche never got over it, and explaining it to himself prompted some of his strongest work. Betrayal was a fact of life to those who knew Wagner.

Facebook Twitter Pinterest Cosima Wagner, first wife of Hans von Bülow, whom she left for Richard Wagner. Photograph: Popperfoto/Getty Images

The strange thing is that many of those he treated so shabbily did not care. The conductor Hans von Bülow accepted as his own Isolde and Eva, the children he knew to have been born of Wagner’s adulterous relationship with his wife, Cosima. When she finally decided to divorce him, Bülow wrote to her: “You are determined to devote your life to a man much greater than myself; I must admit your choice is right.” When Wagner died, Bülow wrote that his own soul had “died with that fiery spirit”. Even the Jewish musicians he maligned remained faithful. Demiurgic is probably the right word for Wagner. The poor deluded King Ludwig of Bavaria, who made the composer’s fortune, broke his engagement to his would-be wife, the Duchess Sophie Charlotte of Bavaria, explaining that Wagner was the god of his life from whom he could not be sundered.

On the question of becoming Wagner, Callow astutely recognises that his unrestrained talking and writing was not simply egomania but “was how he engaged his creativity”: “He needed to dramatise himself as an artist.” Wagner was, as his operas were meant to be, a Gesamtkunstwerk, a total, all-embracing work of art: a great work of his own imagination that he performed endlessly. He could not stop himself. He read all his libretti and tracts aloud and banged out his scores on the piano to whoever would listen. Those who listened were entranced or appalled. He had views on everything and wrote voluminously. His works in the complete edition run to 12 volumes, not counting his scores. He wrote about how his operas were to be performed. He wrote about religion: his art would, he hoped, “save the spirit of religion by recognising the figurative value of the mythic symbols which the former would have us believe in their literal sense, and revealing their deep and hidden truth through an ideal presentation”. The Ring and Parsifal were a sort of new sacred dispensation. He wrote about the climate – the “brotherhood of artist-men” would strive “toward a common pact with, mother nature”. And most notoriously, he wrote about Jews.

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Wagner the man and his work made their own worlds, which is why it is so difficult and perhaps misguided to try to tease out his views on particular questions, or ask whether this or that character is meant to represent Jews in a bad light, or whether Wagner betrayed his true self by embracing Christianity at the end of his life. One of the few times Callow is not surefooted is when he speculates that Wagner would have despised the Nazis, “loathing as he did both nationalism and imperialism”. One could equally say that he would have loved the pageantry and the sycophancy of Hitler and been unhappy that many of the party faithful did not share the Führer’s enthusiasm for worship at the Bayreuth shrine. Wagner would have liked the myth-making of National Socialism, the creation of a fictive past to create a fictive future, something that he and Nazi ideologues shared. It is perhaps this capacity of Wagner’s – this ability to imagine cosmic change – that attracted Lenin to Wagner just as it was Mikhail Bakunin’s apocalyptic vision that attracted Wagner back in the composer’s bomb-throwing revolutionary days.

Back to the question that this short book is well on its way to answering: Wagner was and is so controversial before and after his appropriation by the Nazis, before and after 19th-century radical antisemitism led to the Holocaust, because art-making and self-fashioning on the scale on which Wagner worked are terrifying, at once attractive – drug-like, dream-inducing, mesmerising – and repulsive. Few of us are comfortable travelling so near the gravitational field of a man “who had access to parts of his psyche that most nice people hid from themselves” and who created from such a murky source dramas and music of horrible beauty.

• Being Wagner: The Triumph of the Will by Simon Callow (Harper Collins, £14.99). To order a copy for £12.74, go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min. p&p of £1.99