The movie depicts the life story of South Indian actress Silk Smitha, who died tragically after making her name in popular erotic films.
A source told Mid Day: "In the romantic song that will be shot at Ramoji Rao Film City in Hyderabad, they will be required to lock lips to bring out the love and intimacy their characters share.
"Their chemistry needs to be very natural. So Milan decided to schedule the 'sexy' portions for later."
Hashmi is famously known as a serial kisser because he frequently appears in intimate scenes. However, he recently claimed that he would prefer to be known as a serial killer as he hates his industry nickname.
The insider added: "[Vidya] has never played such a character before. So she's working hard to make sure that not the slightest awkwardness comes across on screen."
Balan has reportedly been asked to wear "butt pads" in order to convincingly resemble Smitha.
In April, she was also criticised by director Milan Luthria for being uncomfortable about showing off her body in the role.