An enormous dose of VFX will be used to make it look as if the actor has sweated it out in the gym day in and day out to expeditiously to get those fascinating abs.
The above pic is taken from Salman Khan ‘Ek Tha Tiger’ film. If you notice the working still (top), Salman Khan is pretty much in shape, but doesn’t have those lethal muscular cuts on his tummy that made the gals go weak in the knees and guys go green with envy. But, after those magical VFX effects, Sallu Bhai tummy transformed into a piping hot piece of iron hard abs ( movie stills at the bottom).
Now we know most of the times the amazing abs that we see onscreen is nothing but illusion. Thanks to technology!