Gerald Finley, baritone

A great opera singer is an artist who can allow an audience to sense the essential condition of a character's soul. The performer strives to be easily understood by both vocal projection and acting talent, to convey a tender thread of fragility, or stormy soul, confusion in dilemma, or broken-hearted revenge. He or she may have a beautiful voice but if it is not connected to a passionate heart, which the listener can almost feel within themselves, then the passion will only be larynx-deep and no amount of vocal talent will move the audience, and they would remain a circus performer.

Humphrey Burton, broadcaster

I remember meeting Maria Callas when she visited London in 1958. She was singing in La Traviata, and as I remember it critics were lambasting her for the scrawniness of her voice. "Violetta is a dying woman," was Callas's explanation. And she had the composer on her side: when he was casting Lady Macbeth, Verdi wrote explicitly that he didn't want a beautiful voice for the role: what he was looking for was personality, charisma, a woman who could act with her voice. Working on BBC's Cardiff Singer of the World coverage, the most memorable occasion was 1989's "Battle of the Baritones" between Dmitri Hvorostovsky and Bryn Terfel. Both are blessed with mellifluous voices, excellently trained (think of the honey in Terfel's Serenade from Don Giovanni or the miraculous long line of Hvorostovsky's Posa in Don Carlo) but it's what they do with their instruments that makes them great artists: their body language is all part of the mystique and they have as much concern with the text and the drama as they do with the music.

Mary King, vocal coach

A beautiful voice is essential, but it can never be enough alone. It takes years to learn to support it so that quality isn't impaired as the singer projects over an orchestra without a microphone. Singers need to understand the text, not merely to sing foreign languages correctly - French, German, Italian, Russian, Czech for starters - but so that they can express and colour the detail therein. Whatever the technical demands of a piece - louder, faster, smoother, more angular - they have to be taken as a given in the moment of performance, when all that matters is the communication of music and character into the auditorium. All singers now expect to have demands made on their acting, and must develop total physical freedom. Without the voice, you can't start training, but without the rest of these things, you can't have a career!

David Pountney, opera director

A beautiful voice is absolutely not enough – it may even get in the way! Just as important are musicality, phrasing, diction, understanding, energy, intensity, charisma and character. "Understanding" relates to emotional truthfulness, life experience and above all imagination. "Character" is the quality that makes a performance individual – this may even include moments of deliberate ugliness. Singers frequently sacrifice diction for what they imagine is "beauty", but intensity of diction is precisely what punctuates and provides a counterpoint to beauty, bringing intensity and meaning into play. Relentless, unmodulated beauty is bland and frequently self-indulgent. Truth is greater than beauty in a singer, as in life.

Ailish Tynan, soprano

A beautiful and interesting voice might just about get you out of the starting blocks in the marathon to becoming a great opera singer, but passion, emotion, dedication and a unique ability to communicate are what make opera singers truly special and are the reasons we queue to see great artists of our time long after the boom of youth and beauty of tone has faded from their voices. They engage and enthral us in ways that a beautiful voice alone never could.

Tommi Hakala, baritone

You need intelligence to make music and to act; nerves to survive the pressure and travelling, to cope with the lonely, endless work; and discipline to live a healthy life. And to quote a dear colleague: "Above all, it takes a hint of too much desire to be on stage, and another hint of too little self-criticism."

Katarina Karnéus, mezzo-soprano

A great opera singer is someone who dedicates their lives to master the communication of music, text and emotions through the singing voice. It's an ongoing commitment: you have a responsibility to never consider yourself a complete artist, to always be curious and willing to learn more. A beautiful voice can move you through just the sound, but it can leave you indifferent if you don't feel that the emotions are genuinely coming through the text, while a less beautiful voice can move you to tears if the emotions are genuine. Young singers today should all learn the art of "bel canto" or the old school of singing. It takes many years to master – your whole body is the instrument, but when learned you are free to connect your mind and spirit and express your inner emotions through a voice that moves the listener and every word can be heard and understood. The sung voice should mirror the soul of the singer.

Petroc Trelawny, broadcaster

More directors are coming to opera from a theatre background – people such as Nicholas Hytner, Katie Mitchell and Michael Grandage. They want artists who will bring ideas to rehearsal and help develop their on-stage persona, so a broad cultural hinterland is vital. As well as delivering the notes, we now expect singers to have a full understanding of what they are singing about, the motivations of their character, so being a good linguist is important. And they need to be cultural entrepreneurs. The big breaks at major houses rarely come for all but the luckiest; the confidence to put on an opera yourself, create original work or find ways of bringing the art form to new audiences will for many be the mainstay of a satisfying operatic career.

Danielle de Niese, soprano

Of course a prerequisite is a good voice, but to be called "great" you have to be an excellent interpreter of the music and text. It is the choices we make as musicians that allow us to shape and create a personal interpretation of the music on the page. To me, being a great singer means being able to communicate not only through the voice but through the face and body. When you share your interpretation, you give a part of yourself. To be great, you need to be willing to take this risk.

• The 30th Cardiff Singer of the World final is broadcast live on 23 June on BBC4 and Radio 3 from 7.30pm.