The scene is about one of the protagonist who was until now, more a cold than a warm person. However she's not a sociopath, she just mostly hides her emotions and tends to push people she cares about away. Now the scene is very important. Her sister was kidnapped and she meets the other protagonist on a hill.
How do I write a scene where she gets really emotional and sad, while not writing her out of character?
I think the best example of this I've encountered is in the book/movie Remains of the Day. The main character is a butler, whose chief trait is total emotional repression. Nevertheless, the author skillfully portrays his emotional state.
If my memory serves me, there's one crucial scene where the butler learns that the housekeeper is leaving. His feelings for her are deeply hidden, even from himself. The only clue to how he really feels is when he goes to fetch the master of the house a bottle of wine --he drops it and it shatters.
It's a small moment, but we've come to know this man as a consummate professional of machine-like perfection. The thought of him dropping anything is absurd. The breakage of the wine bottle lets the reader know how much the housekeeper means to him, even if no other character observes it.
One possibility is to show the character hiding her emotions in the earlier scenes. So readers know she feels emotions, and has reasons not to show them.
Then when she feels emotions that she cannot hide, the readers will know that her outwardly emotional reaction is a big deal for her.
For additional ideas, listen to the "Showing Emotions" episode of Writing Excuses (season 9).
The real challenge here is writing the earlier scenes. A character might not express emotions, but they had better feel them, or the reader will have a hard time caring.
If you look at a book like The Curious Incident of the Dog in the Night-Time, the main character is autistic and has difficulty connecting with other people; however, he feels strong emotions, and this places the reader inside the character's head.
Likewise, a sociopath can still experience a wide range of emotions. In A Clockwork Orange, the main character is very emotional. He's just completely lacking in empathy.
I'm more of a show-er than a teller. If I'm way off base, perhaps you can extrapolate.
Arielle saw him coming from a quarter mile. His relaxed gait and flowing hair reminded her of an innocence now lost to distant memory. As he got closer, Orion smiled and waved.
"Does nothing bother you?" Arielle pointedly asked.
"Well, I, I guess I was just trying to be friendly," replied Orion. "I know this is a big deal; I wasn't intending to making light of the situation."
"Sure, but you have a tendency to not take me serious--anything serious! Sometimes I don't even know why we're friends!"
"Listen, Arielle, I mean ..."
Closing her eyes and taking a deep breath, Arielle tried to steady herself. She was doing it again. She knew she was doing it again. It had to stop; she needed to let someone in, and Orion was definitely the one most qualified. Her lower lip quivered and she briefly sniffled.
She didn't even think she knew how to cry; she could never even remember doing so. But suddenly, her arms were around his neck, and she wept into his shoulder. "I'm sorry. I'm so, so sorry."
The key to a believable emotional expression is the character's vulnerability, which requires sincerity and risk. An insincere statement may move the story through manipulation of other characters, but it won't be seen as emotional. A statement without risk isn't pivotal in any interesting way. Risk creates tension, allows another character to temporarily take control, and for the story to move forward.
How you can achieve vulnerability depends on in whose voice the story is told, and what access the narrator (if any) has to the internal life of the characters.
If you have an omniscient narrator, or the story is told from the first-person, you can show a conflict between the character's internal state and their external behavior. When the external behavior matches the internal behavior, we have sincerity. When that is accompanied by risk we have vulnerability. and a credible emotional expression.
