After teaming up for the hit Irish action-comedy The Guard, actor Brendan Gleeson and writer-director John Michael McDonagh made the unexpected decision to set their next film around a priest.

Brendan Gleeson Brendan Gleeson [L] and Kelly Reilly [R] in 'Calvary'

"After we made The Guard, we were talking over drinks about the idea of making a film about a good guy for a change," says Gleeson. "At the time there was a lot of talk about priests accused of paedophilia, but what if this one was innocent?"

"Then you wake up in the morning with a hangover and you've got to write it," laughs McDonagh. "But it was interesting because most films are driven by the villain, or maybe an anti-hero. So the solution was to have this good priest and then make everyone around him be bad or conflicted!"

More: read our review of Calvary

Both admit that filming Calvary was a challenge. Gleeson says that getting his head around a darker character is easier, because he can separate it from himself and flex new acting muscles. "This film had the opposite effect," he says, "taking me back to my childhood. I had to access a belief in the purity of goodness, something that's in essence childlike, that Father James is trying to preserve. For me that went beyond Catholicism; it was about keeping the faith in goodness and hope as things to aspire towards."

McDonagh agrees that the film is about much more than religion. "It's an anti-authority film, really," he says. "It's about standing up to anyone who tells you what you should do and think. People are always going to seek spirituality of some kind - that's about being human. Whether religion will always be here is another question."

More: Is Calvary the best movie of 2014?

Watch the 'Calvary' trailer: