"Sung a song and composed it for 102 Not Out. Rehearsals on with the great Ganesh... its 'BAADDUUUMBAAAA'! Trust me," Bachchan tweeted.
"One could sit inside the portals of a studio for hours just jamming and having the time of your life. Those moments are special and precious. But with that comes the fear of the shooting of it and the apprehensions of its performing. Never a dull moment in our lives!" he wrote on the
blog.
The film also stars veteran actor Rishi Kapoor.
Amitabh Bachchan's character will be seen as a 102-year-old father to the 75-year-old character of Kapoor in the film, directed by Umesh Shukla.
Billed as a most unusual father-and-son love story, the film is based on playwright Saumya Joshi's well-known Gujarati play by the same name.
The film will release on 4 May.