The actress, who will be seen with Khan in Ready, said that the actor does not talk through scenes before filming commences.
Thottumkal told IANS: "Salman is spontaneous. You have to be very alert while the camera is running because you don't know what he will come up with and what you will have to do as an appropriate thing.
"There is not much of a discussion before the shot. So you have to be ready with whatever he throws at you during the shot."
The star said that the Dabangg actor works in a different manner to her Ghajini co-star Aamir Khan.
"Aamir is somebody who will sit with you and the director before the camera starts rolling. He discusses with you various ways of doing [scenes] and finally decides on one," she explained.
The first look of Ready was recently screened at the Cannes Film Festival.
Trade analysts have said that the film is expected to open well at the box office when it releases today.