An unsettling psychological thriller about a bored 41-year-old suburban mom? You don’t see too many of those around these days, or ever, but Blood Pressure is a movie that breaks a whole bunch of rules, a lot of them behind the scenes.

“No one knew the story until they actually went to see the film,” actor Michelle Giroux tells the Straight, calling from her family’s cabin in Muskoka, Ontario. “The guy who played my son, Jake [Epstein], he came up with his own theory, which is that I was having an affair with a cop. But, really, he had no idea. Nobody did.”

For the record, he was wrong. In her lead role as Nicole, even Giroux was in the dark about her own character’s unpredictable trajectory, which unfolded for the actor as shooting progressed. When it opens, the middle-aged mother is standing in her kitchen reading an anonymous letter that describes her vanilla Richmond Hill, Ontario, world in sensitive if somewhat creepy detail. “I know you’re a good person frustrated by the limits of your life,” it says. “I’m writing to make you an offer. I want to help you get what you are after.”

Deciding to secretly pursue the “one-sided correspondence”, Nicole finds herself being sucked into a plot that’s either dangerous or liberating or both, complete with visits to the gun range and the odd instance of break and enter. All this while she holds down a miserable job as a pharmacist and continues to make bagged lunches for her increasingly suspicious husband (Judah Katz) and occasionally rotten teenage kids (Epstein and Orphan Black star Tatiana Maslany).

“Every time we shot me reading a letter, that was actually happening for the first time,” Giroux explains, chuckling softly and murmuring “exactly” when asked if the process felt more like a parlour game than a film shoot. “So it was very easy to get swept up in the story. I couldn’t wait to get to work each day and find out what was going to happen next.”

With director and cowriter (with Bill Fugler) Sean Garrity being the only guy on-set with the whole story in his head, the actors would be given their own private conferences, and the rest was left to the gods of improvisation. Gradually, Giroux says, the cast found themselves “exploring and finding our shortcomings as a family”. The payoff is an impressively true domestic drama attached to a wild premise that hooks you from the first frame, like Allan King’s A Married Couple as remade by Claude Chabrol.

“The actors were constantly saying, ‘Are you sure this is all gonna cut together?’” Giroux says. “Because it was improvised and nobody knew what the heck was going on, and Sean said, ‘Believe me, it will, it will, it will…’ ” Doubling up as the film’s editor, Garrity actually assembles a remarkably efficient 90-minute narrative out of his unconventional enterprise.

Not so efficient was the production itself. “Yup. Lots and lots and lots and lots of unit moves,” is how Garrity responded to an email from the Straight inquiring—just to make sure—that the whole thing was actually shot in sequence. Which just piles even more inspired madness on this tiny-budgeted Canadian feature.

And yet it ran for six weeks in Toronto, has opened in a slew of other cities, and is now coming to Vancouver on Friday (June 28) for a week. “I was just excited to see it on a big screen at some point. I thought we’d have a movie night in my basement,” Giroux admits when asked to consider the film’s initial prospects. If that’s not incredible enough, consider that it’s also her first-ever feature. She credits her friend and costar Jonas Chernick—whose character brings a few essential twists to the story—for putting her in Garrity’s sights. But the real credit goes to Giroux for possessing something like Nicole’s hunger for adventure.

“Sean only told me so much about my character and said the whole film will be improvised, and I said, ‘Yeah, I’m up for it. Why not?’ ”