The Harry Potter star played local village priest Geraldine Granger in the long-running British sitcom, but French admits she was worried audiences wouldn't be amused by such a holy lead character.
She tells BBC Radio 4, "I thought, 'How on earth do you play a central character who's so blooming good?' I thought, 'Where are the flaws? Where is the monster in this woman?' That's what I understand comedy to be."
French also reveals she would have preferred to take on the role of the vicar's dim-witted sidekick Alice, played by Emma Chambers, but show creator Richard Curtis refused.
She adds, "Alice was a very funny character from the off. But anyway, he (Curtis) wouldn't let me play it."
The show proved to be a massive hit and ran for 13 years before it came to an end in 2007. The Vicar of Dibley also attracted a huge celebrity following and featured cameo appearances from stars including Johnny Depp and Kylie Minogue.