It's quite funny how no one pointed fingers at so many other posters that have been wayy more provocative! dirty picture poster is one recent example...now, why don't you ask how Vidya did it? How can she freakin' imagine Tushar as handsome (even get intimate with him in the actual movie)? Or, how can she romance an old guy like that and get all "turned on" by him?
Ofcourse, I wouldn't have pointed this specific example out had you taken a more general approach to your discussion...you basically only pointed finger at this poster...your conclusion: Shruti can't act, so the eff does she effin' imagine Dhanush (an ugly guy in your opinion) as a love interest? What troubles she has to go through...
well, dearie, there are a few things:
-Beauty is pretty subjective ...have you seen Dhanush's wife? She is quite good looking...so why did she choose to marry him of all ppl?
-And as everyone else has already pointed out, its called acting! To play a certain character, actors have to mould themselves and fit in that character's shoes...think how that character would think...as in live the life of someone else. But how does Shruti do it when she can't act (in your opinion)? Probably, she isn't as repelled by him as you seem to be and has no problems "imagining" him as her love interest...
PS: Before you jump to baseless conclusions, Vidya is a fav. of mine.