There are different ways to act, not just one way. So there is no right way.
Regarding the claim, I can't comment on it because sensational journalism is always about creating sensations or scandal. What I would comment is Deepika, as Sandhya was a bit stiff in the beginning but now she comes across as a believable young girl, who is gently spoken and highly intelligent.
If Deepika acts Sandhya like this, its because this is how the character of sandhya is. I also would like to point out that Deepika has stage experience which is different from TV acting, and would explain her initial stiffness. Like everthing in life, once someone gets used to something they truly shine.
if that doesn't convince you think back to your first day at school/college/job - you want to make a good impression,get lay of the people and environment so your subdued a bit and then slowly as you get comfortable your true personality starts to shine through. Well I would say Deepika's true acting is now shining through. Love her recent range of expressions, compassion, sadness and her intate goodness just bursting through.