The real-life drama, which is directed by McQueen and is up for nine Oscars, stars Chiwetel Ejiofor as Solomon Northup, a free man who is forced into slavery.
Ridley, whose adaptation of Northup's memoir has been shortlisted for an Oscar, admitted he worked on the script for years "without taking a penny", only getting paid when filming actually started.
The US writer said: "There was no development money whatsoever for this project, so I said I'd take it on as a spec project, which means I'll write it for free."
At the time, Ridley was uncertain whether the film would get the green light because the subject matter was "difficult" for American audiences to tackle.
Brad Pitt's production company Plan B backed the project, which helped get some financing from various film studios.
"The upside is I get to own that manuscript; the downside is, for what? This was not Transformers, where you can go to a studio, where they say, 'Great. Let's put some big stars in and go'," he continued.
"It was either going to be a script that wowed people and show how powerful that memoir and that story was, and how pertinent it is. That was the difficulty of it. That was something that came to me in a partnership but ultimately it was a choice of four year's work without taking a penny."
Ridley, who was also one of the film's executive producer, said he eventually did get his "pay cheque the moment they started filming". He added: "In the end, it worked out brilliantly."
The writer, who also penned Three Kings and Red Tails, admitted he is still in disbelief over the film's numerous Oscar nominations.
"It's been odd. I've had a disconnect. It feels like the announcement happened about two or three years ago, when I know it was just days ago," he told the Press Association.
"I am very thankful of the recognition, of the acknowledgement. To make a film that's good is hard, to make a film that's wonderful is difficult. To make a film that is probably one of the best years for film in America, around the world, and have people acknowledge it, it has not yet sunk in."
McQueen was a "very passionate individual", Ridley recalled.
"We had breakfast and he was very demonstrative about trying to explore the slave era in American history, and very curious why Americans had not explored it and why it was a difficult subject matter," he said. "I talked to him about the studio system and that it would be hard to get the money. He was very passionate."