"The Lord of the Rings" actor stars in and directs the new drama, about a man learning to live with his ageing father as he battles dementia, and he reveals no one, apart from his fellow producers, knew that funding was so tight when they started the shoot.
"Falling took me two, three times [of pitching] really to finally get enough money," he shared on the "Armchair Expert with Dax Shepard" podcast.
"When it came right down to it, I'd hoped for a seven week shoot, I ended up with five, and we started shooting knowing - or I knew, I didn't tell the actors - that we had enough [money] for two weeks and we were gonna keep looking. You know, typical thing in movies."
Luckily, the rest of the funding came through just as the existing finances were running out.
"Two weeks in, my co-producer came over to me and said..., 'You can finish the movie... You can finish shooting'," he recalled.
"I said, 'Of course I'm gonna finish shooting..., it's going really well'. He goes, 'No, no, you have the money', and I go, 'Oh, oh!' I'd completely forgot. But sometimes you have to just go for it."
Mortensen, who also wrote the screenplay for "Falling", admits he had been yearning to make his own movies for years, but they were never picked up.
"It wasn't for lack of trying," he admitted of waiting to make his directorial debut. "First time I think I tried to get a movie made that I wanted to direct from a screenplay I'd written once; it was about 25 years ago. I've tried many times with different screenplays I've written over the years."
"Falling" also co-stars Laura Linney, Lance Henriksen, and Sverrir Gudnason.