"Idiots think unconventionally and they listen to their heart while taking a decision. 'Idiots' means 'I do it on my terms', " Hirani told reporters Friday at an FM radio station here.
He came to the FM office with music director Shantanu Moitro, lyricist Swanand Kirkire and playback singer Suraj Jagan to promote his film and was pleasantly surprised to find a birthday cake waiting for him there.
"I thank you all. It's really a pleasant surprise for me, " said Hirani.
"3 idiots" is about three engineering students played by Sharman Joshi, Aamir Khan and R. Madhavan and their experiences on the campus.
The director revealed that whatever is shown in the film, most of it was taken from his college days and that viewers can easily identify with it.
"Whatever you see in '3 idiots' is taken from our life. Some parts of the film are so natural that one can easily identify with what he or his friends did since the beginning of their college life, " said Hirani.
"Whenever we write a story, either we copy from a film (laughs), take from a novel or dig into our personal experience and scoop out moments from it, " added Hirani.
Hirani also revealed that when he went to speak to Aamir about the role, he was shocked because the actor, who was shooting for "Ghajini" that time, was sporting a six-pack abs.
"When I went to discuss the character, I was shocked to see his huge built, but he assured me that he will look much younger than he has ever looked. In three to four months, he toned down his figure and became
lean, " Hirani told.
Hirani says that when he read Chetan Bhagat's "Five Point Someone", he thought of making a film on hostel life. But the story changed in due course of writing.
"The book had very good anecdotes but there wasn't any plotline. While writing the story, the plotline went on changing and it ended up with a completely different story, " said Hirani.
Hirani's earlier directorial ventures -- "Munnabhai M.B.B.S" and "Lage Raho Munna Bhai" -- had a thought provoking message.
Asked if "3 idiots" is having any message, he said: "The message in this film is you study to be eligible, success is ought to come."
Kareena Kapoor and Boman Irani too feature in the film, while Kajol will make a special appearance.
The film was shot at the Indian Institute of Management in Ahmedabad and is scheduled to release Dec 25.
