The trend is changing. That is why Directors and Producers are not in hiding anymore. They together promote the movies. Its a karan johar film, its a rohit shetty film, its a said khan film, etc. I hardly doubt anyone said Humshakals is Saif's film. All were saying it was a Sajid Khan film. (Good for Saif, lol)
As for actress, if a movie fails then people starts finding faults at everything. For e.g. even in Ra.One many were saying movie failed because they over promoted it. if you listen to what multiplex bosses had to say for Jai Ho, they were pretty clear that lead actress is one of the reason for film not attracting enough audience.
In case of Sonakshi, people were excited for Shatrugan's daughter. Wanted was salman's return. The movie did well because everybody liked it. It was after wanted that salman's popularity grew few folds and after Dabangg it sky rocketed.
If we see movies like London Dreams, Veer, they failed.
If a khan movie comes, then people only want best. When they don't get the best opposite lead then they are somewhat disappointed. Star power can get them the first 3 days, then it really depends on other factors as well. Lead actress is one of them.