Kaho Na Pyaar Hai
Before deciding on making this movie a launchpad for his son, Rakesh Roshan, who had worked with SRK earlier in Karan Arjun and Koyla, wanted him to act in Kaho Na Pyaar Hai. SRK turned down the offer as Koyla didn't do well and he was not sure about KNPH's success too. Rakesh then casted his son Hrithik Roshan and the film went on to create history at the box-office and won several awards.
(Also Read): Movies Rejected By Akshay Kumar
Lagaan
Ashutosh Gowariker first offered Lagaan to Aamir, who initially didn’t want to produce the film. Ashutosh then approached SRK for the role but SRK turned it down fearing that the film may not do well. Ashutosh then convinced Aamir and the rest is history. Lagaan went to be nominated at Oscars.
Munnabhai MBBS
Post the release of the movie, it seems that the role was written with Sanjay Dutt in mind, however Rajkumar Hirani had offered the movie first to SRK but for some reason it didn't work out and the role went to Sanjay Dutt.
3 Idiots
Rajkumar Hirani wanted to cast SRK for the adaptation of Chetan Bhagat’s book Five Point Someone into a film. SRK didn't do the movie for reasons best known to him and the role then went to Aamir Khan.