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.
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.”
Queen Elizabeth II told a Paddington Bear screenwriter that she was acting all of the time. Photo / Getty Images
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.’