Margot Robbie as Sarah

in trivia it states she was considered for sarah along with brie larson before Clarke was cast (she had the advantage of having worked with the GoT director. larson was virtually unknown then but Robbie was hot from Wall Street but TG probably started casting just before that was released - in hindsight both were lucky they missed out TG was considered a huge disappointment and probably enabled them to do other stuff that helped make their name but at the time i imagine both were pretty gutted not to get cast)

Robbie is actually a similar height to Linda Hamilton about 5ft 6 (whereas Clarke is a tiny 5ft 2).

i guess shed have looked similar to how she did in Z for Zachariah (same year as TG so she probably had to do that instead)
http://www.elmulticine.com/imagenes/noticias/13/margot-z-zachariah.jpg
https://www.indiewire.com/wp-content/uploads/2015/08/z-for-zachariah-chiwetel-ejiofor-margot-robbie-still7.jpg