The English tenor's voice is like none other: some find it luminous, agile, haunting, ethereal; for others it's too wan, too mannered, too boyish and 'churchy'. His delivery is acutely (some might say garishly) dramatic, and likewise inspires mixed responses. I find his voice beautiful, given it is applied to the right repertoire, but it is his way with words - a combination of intellectual dexterity and explicit emotional involvement - that, for me, makes him a truly enthralling performer.
Bostridge acknowledges his mannerisms are not to everybody's taste. "I move around a lot when I sing. I just do. I have had adjudicators and singing teachers tell me I shouldn't, but I am not about to stop now.
"Moving is the way that I feel expression. The other day I was teaching and noticed how many young singers stand in a way that is detrimental to proper expression. Singing is about acting, and it is important to recognise that."
Ever since he diverted his career from academia (he has a PhD in the history of English witchcraft) into full-time singing, Bostridge has trodden his own musical path.
He does not play an instrument, did not go to music college and has no formal training in music theory beyond O-level.
"But I am not self-taught," he says, correcting a common misconception. "I was helped by a lot of excellent teachers along the way. I developed weird habits at the beginning, and have spent a lot of time trying to get rid of the ones that interfered with vocal production. Others I am not so bothered about."
He made his operatic debut nearly 20 years ago at the Edinburgh International Festival, singing Lysander in an Opera Australia production of Britten's A Midsummer Night's Dream directed by Baz Luhrmann, and Britten's operas have remained a staple ever since. Stravinsky, Monteverdi and Mozart also suit his voice, and he plays the role of Caliban in Thomas Ades's opera The Tempest with touching nobility.
But it is in smaller settings that Bostridge's extraordinary talent for words - for declaiming them, for unpicking their subtlest layers, for inhabiting the characters that they portray - really comes into its own. And it is in song recital mode that he makes his visit to this year's Edinburgh International Festival.
Teaming up with the German pianist Lars Vogt is, he says, "a very intimate experience; often I find myself leaning into the piano and just listening to what he is doing". Together they perform songs by Brahms, Schumann and the American composer Charles Ives.
The German songs (Schumann's Opus 32 set and Brahms's Kerner Lieder Opus 35) date from their composers' mid-careers: what is interesting about that, says Bostridge, is "how extraordinarily differently their careers had developed by that point".
"Schumann did not write any songs in his youth - his piano pieces of the 1830s are full of literary references, but his passion for the voice hadn't blossomed yet.
"Then in 1840 he finally married Clara and wrote masses of songs that abound with emotion. Brahms, on the other hand, wrote songs throughout his career, and they often show a more direct, more intimate, perhaps less intellectual side than is always evident in his instrumental music."
Is it difficult to convey that intimacy in a space as big as the Usher Hall? "Not really. Our job is to shrink the space. Often the sound from the stage is best heard up in the balconies, so it is just a question of making the audience feel close. And that is about emotional conviction."
Bostridge says that when he sings Lieder he becomes the protagonist in his mind - an embodiment of the poet's character "in order to create a unified dramatic thrust through the songs. Sometimes I imagine a narrative that ties it all together: nothing elaborate, just something to hang the drama on."
The Ives songs are a bit of a departure for Bostridge, and likely for much of the EIF audience. Ives (1874-1954) is probably best known for his irreverent, sometimes illogical, strangely poignant sonic montages of American life at the beginning of the 20th century. Military band marches, hymns, European modernism and popular parlour tunes all mash together in his orchestral music, which is often brash and beautiful all at once. He also wrote songs throughout his life: he self-published a collection of 114, the earliest of which laments the death of his family cat.
The mood lapses from sentimental to silly to sad - "they're extraordinarily epigrammatic," says Bostridge, "some as short as 30 seconds and quite funny, some that include speaking, one with a blast of whistling. They're little mood vignettes."
Ives, like Bostridge, had a non-musical first career. He was an life insurance man, partner of the biggest firm in America by 1910, who made a lot of money and proved an innovator in his field (his writings on the insurance business are still studied to this day).
He composed for fun at night and on weekends. "Writing songs was always important to him," says Bostridge. "There were a lot of German émigrés around New York and Connecticut, for whom songs were an important part of late-19th century musical culture."
For Ives, whose experimental side took time to find its feet in larger musical forms, songs were an unbridled expressive release. In the hands of Vogt and Bostridge they should sound it, too.
Ian Bostridge and Lars Vogt are at the Usher Hall tomorrow