They discover their lover is their enemy through decades of family feuds. She takes a knife with her to the grave as not to suffocate if she awakes too early.
Whereas, in the movie they kiss and giggle the entire time. Romeo wears the costume of a knight to show his strength and ambition to do the right thing. The play is about their struggle to contravene fate and create a future together. Although in the movie version of this part, all that the viewer knows is that Romeo is extremely upset from something that was never brought up.
This is bad because it is not very realistic. Romeo says goodbye to his Juliet an drinks the deadly poison, Juliet awakens, sees this and takes his pistol to kill herself. That gallant spirit hath aspired the clouds, which too untimely here did scorn the earth. Romeo fights for his destiny, which is to be with Juliet.
Another major difference in the mood of the play and the movie is in the funeral scene. Prince Escalus regrets this mournfull end.
For example, Juliet wears an angels costume to show her innocence and purity. They are correct.