Ratnam introduced Rahman in his 1992 Tamil romantic-drama "Roja" and has since used him as composer in all his films.
Rahman has also composed the music for the forthcoming romantic-drama "Kadal", written, produced and directed by Ratnam. It releases Feb 1.
"In my 20 years of association with Rahman, I still haven't figured out how he works because he is full of surprises. For 'Kadal', he composed a song a called 'Moongil thottam' when I was out of town. He made me listen to the song after I returned and I was stunned," Ratnam told IANS.
"Sometimes he has played few compositions to me which he claims to have composed on flight. I wonder how did even manage to compose a tune on flight with no instruments," added the filmmaker.
"Kadal" features debutants Gautham Karthik and Thulasi Nair. It tells the story about the lives of Christian fishermen.
The cast also includes Arvind Swamy, Arjun, Lakshmi Manchu and Thambi Ramaiah in important roles.