The actor, 58, plays Reggie Turner, the English author who was one of the few friends who remained loyal to Wilde when he was imprisoned for homosexuality in 1895.
Everett, 59, also wrote the film, a labour of love that he has worked on for about a decade. Firth said: “I knew this was a difficult thing for him to take on. It’s not the first time I’ve worked with a director who is also acting in the film.
“I can see how it can test a person. I thought, ‘What’s Rupert going to be like when he’s tested this way?’ Because I’ve seen the unedifying side of Rupert when he’s not happy.
“I knew it was all doable if you had a particular state of mind. I just didn’t know whether Rupert had it. But he performed extraordinarily. He was not precious about his performance at all.”
Firth added: “When we first had a table read, it must have been about eight or ten years ago. But the way it sounded took my breath away.”