The sketch showed the late Queen taking afternoon tea with Paddington at Buckingham Palace before producing a marmalade sandwich from her handbag.
Farnaby also revealed that the monarch struggled to strike the right tone when delivering one particular line, but eventually nailed it after being advised to imagine she was talking to her grandchildren.
“The director would come in and say: ‘Ma’am, could you just be a bit gentler?’” he revealed.
”And she’d be so sweet and she’d go: ‘I’m so sorry, yes of course.’
”He’d go: ‘Like you’re talking to your grandchildren.’ She’d say: ‘Oh, of course. I’m so sorry.’
”I was saying to him: ‘Just give up, just stop, we’re not going to get it.’ And he said: ‘No, I think just a couple more.’”
The writer said that eventually the late Queen, who was getting tired, got the line right and “everyone was relieved”.
He added: “It was okay in the end. It was really sweet and really lovely.”
Recalling making the sketch on comedian Richard Herring’s Leicester Square Theatre Podcast, Farnaby said it was “a very special thing to be part of”, adding: “That was part of history and great to be there. I was in the same room. It was quite a day.”
But he admitted there were some crossed wires when he talked to the late monarch about her performance.
”I needlessly just went up to her and said: ‘Ma’am, that was fantastic,’” he said.
”She went: ‘Oh, thank you.’ And I went: ‘You’re a very good actress.’ And she said: ‘Well of course, I do it all the time.’