You have mentioned Dharmendra, Amitabh Bachan and Sharukh Khan.
Dharmendra made an image of a He-Man when he became the first hero who put off his shirt on camera in Phool Aur Pathar. This movie changed his image from a boyish to a He-Man and this was the main reason for his success who could beat many people at a time?
Amitabh's voice, height, punctuality and his image of an angry young man made him the star of the millennium. His image of a silent angry man who could revenge or avenge against injustice made him successful.
Shahrukh Khan dialogues delivery, his courage to play all types of role -,even of villain, his looks, his acting and he represented modern youth and their flirting activities made him Superstar or Bollywood Badshah.
All actors have different qualities which were liked by people and they reached on top in movies.
K Mohan @ Moga
'Idhuvum Kadandhu Pogum "
Even this challenging situation would ease
In South India, NTR was seen as Lord Rama and Lord Krishna. If anybody talks about Rama or Krishna we will remember NTR. My friend's father went to the market to purchase a photo of Lord Krishna and he purchased a photo of NTR in the role of Lord Krishna. That is what made him a great actor. The actors who make us think of the original character while acting in those roles are the real good actors in my opinion.