How much do emotions play into murder court cases?

I was wondering about this, after hearing of it a lot. A serious crime such as a murder is committed, then when on trial, sometimes it's the murderer, other times it's an innocent person, but they are quick to be blamed due to how they come off as emotionally withdrawn and almost robotic. Now this was personally scary to me, because that's what I do under stress. I become very apathetic and detached. Despite that, I'm a sensitive, pacifistic person, who believes in the law. I thought to myself, so if I just happened to be a suspect for murder I could go to prison simply because the jury and or Judge, thought I was guilty based upon my emotional response?