Shailene Woodley as Hazel and Ansel Elgort as Augustus.
To answer an additional question: There is not necessarily anything “wrong” with the casting. I don’t know much about either of the actors either, but aesthetically they are just not what I pictured and will take some getting used to.
I saw Shailene on The Secret Life of the American Teenager for a few minutes once and the acting was so terrible that I had to change the channel. But that could just be the writing.
I really do hope that they prove me wrong and that later on I’ll eat my words, which I’m sure I will since John Green believes in the casting choices with all of his heart and I trust him. It’s just going to take some time for me to let go of my preconceived ideas of the characters and accept this interpretation.