Allen frequently plays the male leads in the movies he helms, but he handed over the acting reins to Firth for Moonlight, which co-stars Emma Stone, Hamish Linklater, Eileen Atkins, Jacki Weaver and Marcia Gay Harden.
In the film, Firth plays a 1920s-era, English stage illusionist who goes to the French Riviera with the intention of proving a spiritualist a fraud before she exploits a wealthy family.
"I was trying to imitate Woody. It was just clearly and spectacularly unsuccessful," Firth joked with reporters at a recent press conference in New York.
"It wouldn't have been a fit," he continued in a more serious tone, explaining why he didn't attempt to play the part as he thinks Allen might have.
"It just didn't call for that. It was a wonderfully specific character that belonged absolutely in the culture in the way that it was aimed. I don't know how Woody did it, but I found myself, for interest's sake, when I was preparing just reading P.G. Wodehouse and Evelyn Waugh and [George Bernard] Shaw, just to refresh myself as to how people sounded... Because we don't sound very much like that anymore. I might a little bit because I am a relic, but it has disappeared to a very large extent and it really struck me how remarkably [Allen] had hit it.
So, I play it as I see it or hear it and when dialogue is good and relationships are good, so much of the work is done for you. It just comes to life. It's telling you how to do it and if I'd misjudged that, then I had Woody to make it clearer, what was intended. Good writing just takes you a long way toward it."
Magic in the Moonlight is to open in select theaters Friday.