When the trades announced last February that Albert Nobbs director Rodrigo García was making a small indie starring Ewan McGregor in dual roles, his characters were described as a “holy man and a demon.” But unveiled in yesterday’s announcement of Premieres for the 2015 Sundance Film Festival were the more specific details for Last Days in the Desert that said holy man and demon are actually Jesus and the Devil, “in an imagined chapter from his 40 days of fasting and praying in the desert.”

In the film, Yeshua—Hebrew for Jesus—is tempted by evil while alone in the wilderness. “You can see the demon as the Devil,” says McGregor. “Or you can see him as another side of Yeshua perhaps, an embodiment of his doubt. He’s there testing him into trying to suggest that his father doesn’t love him, that his father’s not interested in his struggle.”

While in the desert, Yeshua also encounters a family that is experiencing its own father/son difficulties. Tye Sheridan (Mud) plays the Boy who wants to leave their bleak desert surroundings and travel to Jerusalem for a better life; his Father (Ciarán Hinds) insists he stay close to home.

Playing Jesus Christ and the Devil was initially intimidating for McGregor, who was immediately won over by Garcia’s spare and intimate 62-page script and the lure of working with Oscar-winning cinematographer Emmanuel Lubezki. It helped to tap into the duality of Jesus and focus more on the human aspects of his character. “I thought, Okay, he’s a man who’s struggling to communicate with his dad,” says McGregor. “I felt actually the pressure come off myself. I’m not playing Jesus; I’m playing a man whose dad is God and he’s trying to speak to his dad. It’s really a film about the relationship between fathers and their sons. In every scene of the film, that’s at the heart of it.”

McGregor played both main characters, with his longtime stunt double Nash Edgerton standing in for those scenes between Yeshua and the Devil. “I played Yeshua with total conviction that he was the son of God and that his father had asked him to go on this path, which would lead him to death—to die for people’s sins—and I tried to imagine what that might be like for a man,” he says. “And then when I played the demon, I was trying to chip away at that conviction. The demon’s trying to drag him away from that.”

That much is very much in line with the Biblical narrative, the story of Jesus being tempted by the Devil during 40 days of fasting in the wilderness, but McGregor is careful to leave the door wide open to interpretation. “It’s not a Biblical story. It’s a story that Rodrigo García invented,” he says. “You could watch the film and not think that it’s Jesus. He could just be another holy guy, a rabbi, walking in the desert, looking for answers.”