Okay, i know this -or part of this- has likely come up a lot but i was wondering how to avoid making characters too emotional and just more convincing. First of all, a character's relative dies in the story, and they are torn apart with grief. Later on they discover an ability that can get them revenge, and they become semi-obsessed with that. But all goes wrong, and they end up captured by the enemy who they want to get revenge on. Then, they are left helpless and the enemy saves them, leaving them torn about getting revenge or forgiving them. It is going to be a fantasy/young adult romance (very loose on the romance) between two teens. Would it be too much to say that these reactions are believable? (plus there are so many twists thrown in it will make your head spin)
I'm not sure anyone can tell you how to make your characters behave so they don't come out as too emotional. you know your characters better and you know what behavior they do that can render them over-emotional. Since they are teenagers, their personalities are hard to predict. I mean there are the strong teens that can think and act carefully or take their pain inside and let it out in other ways, there are those teens that whine and mop around acting like the victims, the saddest in the world. usually if a person is set on revenge from someone who killed their beloveds, they don't mop around. revenge comes from anger not from sadness, so it's highly unlikely that your char would want revenge and whine about their loss.
If you make it work then yes, show how they go from one thing to the next and anything can be believable. Under stress and hard conditions people can go to the extremes but once they cool down they think things over.
^ This When you edit and re-read, you will notice if the characters are not interacting naturally or in the way you intended. The "feel" will either be right or wrong. Sometimes, especially with male/female interactions, I will ask my girlfriend how she would respond or what she would think, in a given situation. Believe me, sometimes that is not a can of worms you will be happy to have opened! It may become clear, that the crazy-sounding (to a guy) stuff your girl says to you is only the tip of the iceberg when it comes to the crazy-sounding stuff she DOESN'T say. She doesn't say it because it sounds crazy, even to her.