Although Sanjeev Kumar played the father, he was actually several months younger than Shashi Kapoor, who played his son.
Mithun Chakraborthy use to frequent a famous tailor shop in Mumbai called "Playboy". Once in 1976, Mithun had arrived at the shop and saw Salim Khan. Salim noticed Mithun standing around and kept observing him. Salim was taken back by Mithun's face. Salim finally approached Mithun and said "Bhai tumhare chehre mein koi kashish hain. Aap filmo mein kyon nahin try karte?". Mithun knew exactly who Salim Khan was and could not believe his ears when Salim Khan paid him that compliment. Mithun told Salim Khan he just graduated from Pune's FTII acting school and was looking for work. Salim Khan gave him Yash Chopra'a address and told him they were casting for Trishul. The needed a boy for the role opposite Poonam Dhillon. Mithun thanked Salim Khan for the help and set out to meet Yash Chopra. Unfortunately Chopra had already signed Sachin for the role.
Amitabh Bachchan was nominated for Filmfare best actor award but he lost out to himself for Don 1978.
Rishi Kapoor was offered Sachin's role but he refused which shocked Salim Javed who told him that it will ruin his career if he refuses their film as there were a big name as writers
This Rishi mentioned in his autobiography
There were several reports that Pam Chopra( wife of Yash Chopra) was interfering with the film's script. This angered Salim and Javed. Pam would sit in on the film's rush's and criticize scenes. Ordering Yash Chopra to change many things that she did not like. What tip the ice berg was Pam telling Yash Chopra to tell Salim and Javed to rework the script . This would make Yash Chopra re shoot the film for 40 days. Not only that, but after the 40 day shooting schedule, Pam insisted on more scene's being knocked off. Salim and Javed could not speak up about Pam's interfering issue because she was a woman and Yash Chopra's wife. Finally Salim and Javed got there revenge. They spoke to the editor of the film and convinced him to delete Pam Chopra's song from the film. When Yash Chopra saw the song missing, he was furious. But Salim and Javed convinced Yash that the film was running to long and taking focus away from Amitabh Bachchan's character. After Trishul wrapped up, Salim and Javed decided that for Kala Patthar there be a clause that there would be no interference from anybody regarding there script.