Haggis revealed the news to New York Magazine's Vulture at the 2012 Revlon Concert for the Rainforest Fund.
"It's about three storylines, three love stories, three relationships in three different cities that combine in a very odd way - New York, Paris, Rome," Haggis said of Third Person, for which he hopes to start production in August.
He revealed that Neeson will play a character who is in a relationship with a party reporter (Wilde).
"I wanted one of them to have a job in New York," Haggis said. "So after writing the script, I thought, 'Oh, party reporting, that would be great'. But literally, it's just that. The character is not based on anyone, or anything like that."
Neeson, who can be seen next in Battleship, is also reported to be in talks for the action thriller Non-Stop.
Wilde was recently spotted "marrying" co-star Chris Hemsworth on the set of racing biopic Rush at St John's Church in London.
Watch the trailer of Battleship below: