The biopic, based on Indian death row prisoner Sarabjit Singh, will be narrated through the perspective of his sister Dalbir Kaur played by Aishwarya Rai Bachchan.
Randeep, 39, who had gained weight for his romantic drama "Do Lafzon Ki Kahani", called his physical transformation a "roller coater" ride.
"From 94 kg of #DLKK to 66 kg for #Sarbjit.. It's been a roller coaster ride in the same year.. Please enjoy your food," he tweeted.
"Sarabjit" also features Darshan Kumar as Awais Sheik, a Pakistani lawyer who left no stone unturned to get justice for Sarabjit.