You know those scenes.
For example, the "There is no God!" scene in I Am Legend. It is so powerful because you know Robert feels like world has given up on him, he lost hope and his frustration has taken over.
Juno - "I'm a planet!" That scene is so powerful yet simple. Juno's just a teenager and she's asking the boy she likes to like her back, but he's so scared and she's terribly hurt. Great acting there.
Titanic - When Rose abandons the boat to go find Jack. When they meet all he can say is "You're so stupid, Rose" while kissing and hugging her.
Troy - When Achilles is calling for Hector, and you know noble Hector is doomed.
In God's Hands - When Mickey gets eaten by the waves. He dooms himself as he paddles on, giving himself over to the ocean.
RENT - I'll Cover You (Reprise). Oh man, serious chills on this one.