"It was early, and the goal was to have Mass before what was typically a 12- or 14- hour filming session."
He said the other thing he did was a simple blessing of each set every time the film crew moved to different locations around Stillwater.
The directors of "Unplanned" have been open about the fact that several bizarre incidents took place during filming involving cast and crew, including a near-death experience involving the film's lead actress, Ashley Bratcher.
"What was interesting is that the cast and crew, all throughout filming, reported kind of strange happenings," O'Brien said.
"The goal was to offer God's blessings upon this project and see if we could prevent some of that."
Though the time O'Brien spent on-site was not very long, he said he enjoyed sitting on the set while the crew was filming scenes for the movie, and to support two of the film's co-directors, Cary Solomon and Chuck Konzelman, both Catholics.
"It was on the set where I met lots of the other pastors of other churches, so I think they put out a call to any church that was sympathetic to the pro-life cause and said, 'come,'" O'Brien said.
The broader population, meanwhile, was told little about the movie until later in production.
"We only announced it here to our people at church when they needed extras for what I think is the final scene, kind of an aerial shot of a bunch of people at the park," the priest said.
"They needed extras for that, so we invited all of our people to go, and a lot of people did."
Now that "Unplanned" is out in theaters, O'Brien said his parish is doing a lot of promotion for the film. They have already bought out two theaters and expect about 200 people to come to see the movie all together.
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Sheryl Lacy, operations manager for St. Francis Xavier Parish, told CNA that she helped to encourage people to come and be extras in the film, even those from communities outside Stillwater.
"It was exciting being able to say it was happening in my hometown, but it was quiet, no one talked about it because not a lot of people knew about it," she said.