Dallas Buyers Club star Matthew McConaughey "drove" the movie, according to director Jean-Marc Vallée.
McConaughey, who won the Best Actor Oscar for his performance as an AIDS patient Ron Woodroof in the film, was said to be the driving force behind the movie's success.
Speaking to People magazine, Jean said: "He became the engine that pulls the train out of the station and brings it to its destination."
Jean, who cast Matthew in his breakout role in Dazed and Confused in the 90s, also gave credit to McConaghey's wife stating that Camila Alves is a huge influence on him.
He added: "[She was there] helping him with his diet and just being there for him.
"[In Hollywood] you'll have people around you who want other things for you, but [Camila] will approach it as, 'What does Matthew want?'"