I feel that RK is already attracted to her because she is the only one to have stood up to him and not bowed down by his good looks or fame.
He tried very hard to bribe and defamed her but she stood firm on her decision. So RK has understood her character to be big on values and LOYAL to relationship no matter what.
RK himself is big on values and LOYAL to relationship but since his father's death, he has lost respect for people starting with his mother who remarried for whatever her needs were at that time.
In Madhubala he sees that same value and Loyalty he believes in but thinks it will not last if put through torture and hard times. When Madhubala endures all torture and hardships and continues to do good deeds, he will fall for her. At the moment he has learnt to respect her for saving his life.
As for Madhubala, she has just glimpsed the human being for a second, as she does good deeds, RK will return the favor hence they will both start feeling for each other around the same time.
Who will show first to the audience, I think it will be RK.