 Sean Maher recalls reading for the part and liking the character of Simon Tam , but that it was Whedon 's personality and vision that " sealed the deal " for him . For the role of Simon 's sister , River Tam , Whedon called in Summer Glau for an audition and test the same day . Glau had first worked for Whedon in the Angel episode " Waiting in the Wings " . Two weeks later , Whedon called her to tell her she had the part . 
