There is always something you're fighting for in a scene. We want to root for you. But we need a reason to care, we need to know that you are just like us. We need to see your struggle, see that you have moments of doubt and regret. You cannot rely on the script to provide a "to be or not to be" soliloquy to explain your inner turmoil. You must express it truthfully and fully while you continue to fight. We want to see that your reason for fighting is human, your job is to make the audience see that your intentions are human. The best example is My Best Friend's Wedding. Julia Roberts is doing a horrible thing (breaking up an engagement) yet we root for her. We want her to win because she expertly portrays a look of terrible heartbreak every time losing becomes a possibility.
When another character double crosses or betrays you in a scene, the easy generic choice is to just do angry. Many actors become one dimensional and sub human by deciding, that "someone did me wrong, I'm pissed". Consider that anger is a secondary response in humans and always stems from hurt, loss or abandonment of some kind. The only way someone could actually make you angry in the first place is if you cared about them, trusted them and had a personal stake in how they treat you. That doesn't go away when they betray your trust. Love lives on. Simply responding with anger is not enough. Adding hurt is not enough either. You must also show through behavior that you do still care. That is the ultimate pain of human existence. That makes you compelling to watch.
You are not a robot. Holding your breath hurts you in many ways. Firstly you need oxygen to survive. So does your character. Not breathing also stops emotion from flowing through you freely and keeps part of your brain focused on yourself rather than something outside of yourself. Your characters breathe, if they are real people. Deep breaths and sighs are some of the best true behavior you have because people are immediately affected by them and they occur naturally when you are stressed in some way. Breathing is not something to plan, it is what you must allow into your acting. It is undeniably human.