Spacey told AFP the project survived against difficult odds to get the big-budget shows on the road across three continents and hang on to big sponsors despite the financial crisis and widespread recession.
"If you have a good enough idea... I was pretty confident we'd be able to raise the money," said Spacey, who does not act in the shows but helped produce them.
"We did, just before the credit crunch, and (sponsor) the Bank of America stuck with us," he said, sipping champagne at the theatre before Wednesday's Paris premiere of "As You Like It".
"Sam and I were always looking over our shoulders wondering if the phone call was coming (to cancel the funding), but it never did."
English-language plays are not rare in Paris, but Hollywood figures of such renown have drawn media attention in the cinema-mad country.
The shows are the second of three seasons of the Bridge Project, a tour using British and American actors launched by Mendes and Spacey, who is an established force in European theatre as director of London's Old Vic.
"As You Like It" opened to a packed house with British actor Stephen Dillane stealing the show as the grumpy but wise outsider Jacques. He also plays the vengeful wizard Prospero in "The Tempest", opening here on April 20.
Mendes was not available for an interview but told French cultural magazine Telerama ahead of the Paris run that he was influenced by another British director known for his international productions, Peter Brook.
Based at the Bouffes du Nord theatre in Paris, Brook has experimented with touring shows and multinational casts, most famously in Mahabharata, his epic 1985 staging of Hindu legends.
Brook wrote in the 1960s that theatre shows need to adapt constantly to their location and audience and a play that brings the house down in one setting can fall flat elsewhere.
Spacey insisted that theatre transcends language and cultural barriers. The current shows have played in New York and are also bound for Singapore, Hong Kong, Greece, the Netherlands and London.
"You could say that even people who speak English don't understand every word that Shakespeare wrote, but you certainly understand the feeling and the emotions," Spacey told AFP.
"Different audiences will respond in different ways... but at the end of the day it's just about exposing the human condition and I think it just doesn't matter where you come from."
"It's my first international tour," Dillane told AFP in his dressing room after the Paris premiere. "Even if you're running for six months in London, every night, every audience is different," he added.
"It just happens organically. The performances change to suit a venue," said Dillane, a top stage actor who played Merlin in the 2004 film "King Arthur" and alongside Nicole Kidman as Virginia Woolf's husband in "The Hours" (2002).
Mendes said that with the Bridge Project he too aimed to "tour with shows that are modified constantly according to the place and the audience," according to comments quoted in French by Telerama.
He has spoken of the Bridge Project as a return to his first love, the theatre. "I learned in the theatre how to tell stories," the magazine quoted him as saying. "How to know when the audience is bored or is enjoying itself."
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