Bachchan, 72, plays an unsuccessful artiste, who lends his voice to a deaf and mute actor played by 'Raanjhanaa' star Dhanush.
The R Balki-directed film released last Friday.
"Reading the reviews on 'Shamitabh' gives credence to the film, the thought and the performance... And that is most gratifying for the artist, many varied colours are painted, but what remains without any doubt is the effort and the execution of that which has been elusive, for most..." he wrote.