The simple explaination for why actors seem terrible in some of the movies they've done and not in others comes to a few things, personal preference, "Do I actually LIKE the genre this actor is in?" A second view may be, "Are the scripts/premiss(SP) worthy of an actor of their talent?" And yet a third, "Do the directors know what they are doing?" To give an example, Mammoth, terrible script/premiss(SP)/plot, at least two great actors!

In answer to your second question, Simon is FAR too introverted to REALLY tell Kaylee how he feels. He's had to close himself off from the outside world to protect his sister and himself, he doesn't want others to get hurt if/when something happens to him.
My post has gone on FAR too long! So I hope I've answered your questions.