I love the change he goes through from old to young in the prologue. When I first sang the role in 2005 at Covent Garden I spent hours in the parks watching old men moving, with a stick, how they react to things. It had a direct influence on my basic idea of Faust. You have to make the character believable visually, not only through the makeup and costumes, but also the movement. But of course the most challenging part is to capture the old Faust vocally.

How do you do that?

He sings with a different color. My vocal teacher used to tell me never to think of the beauty of the voice in that first scene. You have to start with an almost baritonal, rough sound, and then a half-hour later you have a high C and a gentle sound in "Salut demeure," which is very beautiful and elegant. That's the style of the young Faust. It's all about finding the right color to develop the story. You start low and expressive and dramatic, and you can't push the voice too soon, to save the beauty for the rest of the evening. And then you have to find a way to show the degeneration, the decline of the character. When he first meets Marguerite he is enjoying this new young body, and from there he's going down, down, down. The only exception is when he tries to save Marguerite at the end: and he doesn't know how to do that.

It seems at first that Faust is more intrigued by Méphistophélès than afraid of him. At what point does he begin to see the devil's true intent?

In the trio with Valentin and Méphistophélès. Before that he's joking. He starts taking things seriously when it becomes dangerous. When he sees the death of Valentin, that's when he realizes which direction his relationship with Méphistophélès is going.

Last month you sang the Duke in the new production of Rigoletto. Do you see any parallels between your two characters?

Faust is a very honest person. Everything he says he means: or he thinks he does. The Duke is always joking, playing, making fun, directing people to get what he wants. His only honest moment, where he's describing his true feelings, are the three and a half minutes in the second act when he sings "Parmi veder le lagrime." From Faust's first "Rien!" to his last line, he is completely honest. He never pretends to be anything else.