You have to be fit enough that you're not focusing on the physical aspect, so you can let the artistic side through. Not to mention that a cold or flu at the wrong time could cost a night at the Met. But looks alone don't make a character believable. You can't just stand there and sing pretty. Conveying character is as important as hitting the right notes at the right time.
Good singers make using their exceptional instrument look easy.
The audience sees a singer as someone who pours out glorious sound. They don't see all the work behind it. You have to be careful when you are young not to do any harm to your voice by abusing it. She got her first big break when she was called in at the last moment for a role in Berlioz's Les Troyens with renowned conductor Sir Colin Davis. The role was her debut at La Scala, Milan's famous opera venue, and led to a callback to work with the equally renowned Riccardo Muti in a production of Falstaff.