My theory on Arjun is that, at the beginning, he did try his best to be a good actor, he worked hard on his roles and physique- and it showed. He did have success with 2 States and a few more.
Then he fell inlove with Malaika and also he probably realised he's not gonna 'catch up' in rise with RS, RK, Shahid and others and will not have same career path. By Ki &Ka he's realised he'd be a 2nd tier type actor. And I think he really disliked that. He kinda knew that most of his movies will come from his dad and his dad's buddies..or those that will do his dad a favor..
So he kinda let himself go..and let his 'spirit' go.. didn't prep for roles as much..didn't bother with his look as much.. wasn't 'hungry' enough.. so he left space for Ayushman and RR and Vicky to get roles and be praised..
When he saw that others were also advancing well and he was still not getting the roles HE THOUGH he deserves.. then he probably decided subconsciously to just 'breeze through'.. get the job, get the money..and that's it..
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