"My admiration for Mohanlal could never have made me refuse this offer and so I do it for him, " Amitabh posted on his blog.
"Mohanlal has been and still is one of the most incredible talents from Kerala and the Malayalam film industry. I have admired him and his work through many years of my existence in this profession. He has a unique body and face language. You get the feeling that he is doing nothing, yet it results in the most amazing results - natural, without effort and effective. Just such a joy to watch him, " he wrote.
The offer for the yet-to-be-titled project came to Amitabh when he went to Kerala to attend the celebration of sound technician Resul Pookutty's Oscar victory last year.
"It was there that Mohanlal, a guest at the occasion, had come up to my room with his director Major Ravi to put up this idea and this proposal. A few weeks later, they all came to meet up with me in Mumbai to set up all the details for the shoot, " he posted.
The 67-year-old is working for free in the movie.
" (They) put up the inevitable question that most producers do of remuneration! I had laughed at them. What? Charge you for working for three days in a Mohanlal film? An actor I admire? You must be joking! No way, no fees, no nothing. Just tell me when I have to report and where. That is all."
There won't be any language barrier for Amitabh as he is playing a north Indian character.
"The film will be in Malayalam, but my character is north Indian so I shall be speaking in Hindi, with the odd sentence in Malayalam. Now that shall be tough, but effort shall be made, let's see what happens, " he wrote.