The director said Devgn was the first actor whose name cropped up in his mind when he decided to make the film, which is a remake of a Malayalam movie of the same name.
"I needed a star who was in his mid '40s and half of them were out (of the race), including the younger stars. Secondly, I needed someone with very intense eyes and Ajay fit the bill completely. He was the obvious choice for me. He readily agreed to do the film which was very important," Kamat told PTI.
"I cast Tabu as I wanted someone, who is a mother at heart and a very strong cop. Someone, who is both vulnerable and strict. Tabu was my first and last choice," the director said.
Kamat, who has earlier directed Bollywood films like "Mumbai Meri Jaan" and "Force", said he decided to remake a successful Malayalam film like "Drishyam" because he wanted a pan-India audience for such a "fantastic story".
The filmmaker said remaking a suspense thriller may sound tricky but he faced no such problem as he has tried to be true to the original movie and changed only 40 per cent of the script.
"The original film is so good that I haven't tampered much with it. I did not face any challenge as such while making the film. I am aware that there are people, who already know the story and have already watched it, but there is a large section, which has not seen it.
"Drishyam", which also stars Shriya Saran, will release on July 31.