Jonathan Biss: Many of us become musicians in part because we feel out of step with the world. Music makes up for something that is missing. And Beethoven has done that for me. But his personality is so immense, irascible and confrontational that living with him all the time, as much as he made life bearable, he also made life unbearable. And that’s the conflict that eventually tore me to pieces.

Stephen Fry: Perhaps Shakespeare is the closest to Beethoven in greatness and all-consuming emotional range. There are actors who have had breakdowns playing certain characters. Hamlet and King Lear can be incredibly upsetting. There’s a famous film from 1947 with Ronald Colman playing Othello. He murders his wife on stage because the character consumes him. These things make for good stories – and they were told at a time when mental unease was the joke you told about artists. It was part of the artistic temperament, part of their eccentricity.

But now we’re more aware of how dangerous health mental health problems can be. They’re not a joke, but can be frightening and serious. And Beethoven’s music is frightening and serious. I can’t imagine somebody having these issues playing Schubert, profound and wonderful as Schubert is. If you picture the great composers when you die and are in heaven, you’d be pretty sure Schubert would take your hand, put an arm around you and take you off for a drink. Whereas Beethoven would be like Bob Dylan or John Lennon. They’re the rock stars you don’t want to meet because they’d look down their noses at you.

JB: I think piano soloists all feel the pressure to be, if not perfect, at least invulnerable. You’re supposed to come on stage and slay a woolly mammoth and look like it isn’t difficult. We are not supposed to show vulnerability. That’s a terrible burden. Yet the performances that have meant the most to me as a listener are those in which there was some display of vulnerability.

SF: With the performing arts, there are some parts of humanity you can display and other bits that go over the line. When an actor appears on stage playing Uncle Vanya, they can have a false beard, but have their real hair and face. Then that’s Uncle Vanya on stage. But if the director decides Vanya is to take a shower and the actor takes their clothes off, you’re no longer looking at Uncle Vanya’s penis, you’re looking at that actor’s penis. Depiction disappears – because that part of someone’s body is not up for consideration off the stage, as a rule, since we’re all peculiar about that.

There’s an emotional version, too. You can come on stage and show this emotion or that emotion. But if you stab yourself and the blood comes up, people go: “Whoa, that’s going a bit far.” There are limits to the exchange you expect in an artistic experience.

JB: Do feel like you’ve crossed that line before?

SF: I’ve played characters who had a desperate time. Playing Oscar Wilde was a very interesting experience. But actually what affected me more was playing Malvolio in Twelfth Night in London and then on Broadway. Malvolio is rejected and laughed at. “I’ll be revenged on the whole pack of you” – those are his last words. And off he goes on his own. He’s outside the comedy embrace that Shakespeare gives every other character in the play. Everyone except Malvolio is welcomed into a great smiling hug of pleasure. And it gets to you! It’s a strange thing. I mean, you know it’s pretend.

JB: But you’re probably having to access some part of yourself, and that part has to get hurt over and over.

‘I’ve played characters who had a desperate time’ ...Fry with Jude Law in the 1997 film Wilde.

SF: We’re both people who’ve had issues with our mental health and stability of self – and, in my case, mood disorders swinging between mania and depression. Bipolar disorder it’s called. People often ask: is mental ill health, while not necessarily a condition of being able to be creative, helpful? If you were told by a doctor that they had discovered this new amazing gene therapy and that in 10 minutes you could be rid of all your mental health issues, would you say: ‘Yes, please go ahead’? Or would you fear you would be less of an artist without these issues?

JB: I don’t know – I’ve never had the experience of being anyone else! There’s a part of me that recoils from glamorising the artist as someone not quite of this Earth– having special abilities but also being, almost by necessity, irresponsible. I think all people feel deeply and artists come in all personality types. But at the same time, I do think that when you make music, or any kind of art, your whole self is revealed. I feel like the part of me that is anxious is inextricably linked to all the other parts of myself, which probably are essential to my being a musician. It’s not that I wouldn’t want to live without it – I would love to live with less anxiety – but I don’t know that the other parts would remain what they are. It’s a very unsatisfying answer.

SF: It’s unsatisfying because you’re right. One cannot know. I’ve been on stage, or in a television studio, talking while another part of my mind has been saying: “I’m in hell.” You’re dead inside, yet the part of you that won’t allow you not to entertain your audience – timing the joke properly and being cheerful – is alive.

I was on a talk show once and the other guest was Robin Williams. My godson called me up and said: “I really enjoyed you on that. And Robin Williams, isn’t he amazing?” I said: “Yes, he is incredible.” He said: “But he sweats so much.” I said, “Yes, do you know why?” He said: “Was it a condition?” I said: “Yes, a condition of trying so hard.” He may look as if he is just laid back, but he is absolutely trying.

‘Normally I play 80 concerts a year’ … Jonathan Biss on stage at 92Y, in New York.

“Ars celare artem est,” as the Romans said. True art is to conceal art. You are not supposed to show it. When you finish a performance, your heart is going like the clappers. It never leaves you, that hammering nerve thing, and on top of that is the sense of: “Did I fail or did I succeed? Will I fail tomorrow if I succeeded today? If I succeeded today, does that mean I’ll fail tomorrow?” All these stupid things, these voices in your head as you try to sleep at night after a performance, knowing that you have another tomorrow. It is astounding that every artist isn’t certifiable and locked up.

JB: Like so many musicians, from March until September last year, I didn’t get on the stage at all. Then I had two concerts in September, followed by two in November. Normally, I play 80 concerts a year. It was a huge thing that, for all those months, I did not play in front of people. And that time coincided with me trying to finally address these demons. At one of the concerts in November, I was warming up and getting nervous, as I always do. All of a sudden, I realised I didn’t actually have to try to be anything I’m not. And it was the first time I had ever felt that way. It was absolutely revelatory. By the way, I only hung on to it for about three and a half minutes. The urge to be some perfected version of myself dies hard.

SF: Years ago, I was working with one of my comic heroes, John Cleese. He asked: “So what sort of things are you doing at the moment?” I told him I was doing some stage things and that I found them nerve-racking. He said: “You have to remind yourself that the audience haven’t come in, sat down, folded their arms and said, ‘Go on then, entertain me, I don’t believe you can.’ They’ve paid money because they want to see you.” And you think: “Yes, of course. Yes. Why am I thinking that they’re all hostile?” Generally speaking, people have come because they want to. If anything, the audience are perhaps a little more nervous than you.

Your job is actually to tell the audience: “You’re in wonderful hands. You’re going to have a fantastic time. It’s going to be marvellous.” With an actor, that moment starts with the first scene, but there’s that extraordinary thing a pianist does: they sit at the stool, put their hands out ready to play, then there’s that pause. I’ve always thought, ‘What are they doing? Are they clearing their mind or filling it? Are they hearing the music ahead?’ As an actor, you can do the same thing: just let the audience know they’re in for a good time. It’s a way of hypnotising yourself as well.