Peter Shaffer, writer

I came up with the idea for this play after reading a lot about Mozart. I was struck by the contrast between the sublimity of his music and the vulgar buffoonery of his letters. I am often criticised for portraying him as an imbecile, but I was actually conveying his childlike side: his letters read like something written by an eight-year-old. At breakfast he'd be writing this puerile, foul-mouthed stuff to his cousin; by evening, he'd be completing a masterpiece while chatting to his wife.

John Dexter was to direct the first production, at the National theatre in 1979. We'd already worked together on three of my plays; but, while I had great admiration for him, he was a difficult man. He insisted he was due a percentage of all my royalties – including for future stagings even if he wasn't directing. My agent told me not to accept as it could set a damaging precedent. I wrote to John and he sent the letter back unopened. So I went to see him and there was a dreadful quarrel.

Peter Hall, then the director of the National, had directed productions of most of Mozart's operas. He told me, moist-eyed, how much he longed to direct Amadeus, but hadn't wanted to vie with John. I realised I had the greatest Mozartian of all time before me. I decided I would no longer be bullied by John and asked Peter to do the job. But parting from John, who had worked with me so brilliantly, was agonising. We were never friends again.

The only person John had cast was Simon Callow, then a young unknown, as Mozart. During the hiatus, Simon had developed misgivings and told me he didn't think he was right. I reassured him, even though I had no idea then about his talents: I trusted John's judgment. And as usual, he was right.

During rehearsals, I sat in the stalls, rewriting scenes as the actors brought my characters to life. It was a joy to watch. Simon was startled that Mozart could have spoken so childishly and vulgarly; at first, when he said his lines, the cast got the giggles. But we got them all together to discuss the silliness behind the genius, and they absorbed it.

It's not a policy of mine to compare one actor to another, but Felicity Kendal was, in my opinion, the best Constanze [Mozart's wife] of them all. Now, when I think of the character, I see her face.

Felicity Kendal, actor

Being in Amadeus taught me to focus on the play rather than the role. It made me realise how I wanted to work. I was quite an arrogant person in those days, with a firm idea of what I wanted. I would turn down roles – then regret it.

Constanze was a very small part. She's central to all the emotion, but it's not her story; if it hadn't been for the brilliance of the script, I wouldn't have said yes. As it was, I didn't even register the slightness of the part. I just knew I had to be in the play. I think I'd have worked on it even if she'd ended up being written out entirely.

Paul Scofield, who played Mozart's rival Salieri, was a colossus for me and Peter was one of my idols. Every day, I'd wake up and watch the clock, impatient for rehearsals to start. There was no egotistical starriness. Everyone was hyper-sensitive to the play but also hyper-hardworking, although we did corpse a lot in rehearsal – and occasionally during performances.

Peter likes to keep rewriting his plays, and there was a lot of that going on. Towards the end of the rehearsal period, he produced a sheaf of revisions. Paul looked up and boomed: "No!" And that was the end of the rewrite.

I had to dress and undress Callow a lot off-stage: we had to develop a very quick, precise choreography. It was a challenge not to get a walking stick up my nose because he'd decided to flourish it when I was about to button his coat. He would always have a glass of white wine waiting for him off-stage after every performance. I thought that was very civilised.

The excitement lay in being a part of something that was pushing the boundaries. It surprised me how much emphasis people placed on Mozart's vulgarity, since the play was chiefly about people's jealousy of effortless genius as opposed to hardworking talent. And the vulgarity was a way of separating Mozart's genius from his humanity: to show that we can't explain genius, however hard we might try.

It's rare that you know something is going to be a classic when you are still working on it. They talk about the sweet smell of success, and it's true - while we were rehearsing, we could smell it.