To test their chemistry before casting Hardy as a hardworking single father and Patel as a South Asian drag queen with whom he forms a surprisingly deep connection, directors Sally El Hosaini and James Krishna Floyd had the actors perform some scenes from the script.
Hardy recalls how their rapport together was "instant."
"I think it's either there or it's not. I've done things before, I won't name them, where I've tried to manufacture chemistry and spent a lot of time with people."
"But I think it is like when you fall in love, sometimes you meet someone, you hit it off and things go from there. I think if you don't have that on screen, it shows," says Hardy, adding, "But I'm not saying we're in love!"
Adds Patel, 27, "We have such a great friendship now. We even worked out this morning together."
Hardy, a British actor best-known for playing the mutant Angel in X-Men: Apocalypse and Queen drummer Roger Taylor in Bohemian Rhapsody, was excited by the opportunity to tell a story unlike any he'd told before.
"I thought [Unicorns] would be something I've never done before, so I thought it was something that would be a good test for me," he says. "Ultimately, I just thought it was an incredibly beautiful story that I wanted to be a part of."
For Patel, who previously played Mowgli in a touring production of The Jungle Book, making his film debut in Unicorns was his chance to "bring everything I possibly could have to something."
"This is the part of representation that I want to bring forward," he says. "I didn't have that when I was younger. I gave it my all."
Unicorns next plays at the BFI London Film Festival. It's currently seeking distribution in North America.