Restrained for two reasons:
1. Because the cultural mileu of the time and place is a restrained one. Folks in the rural west are taciturn and stoic, not prone to talking about feelings or experiences. The characters had to be true to that reality, and so talking for the sake of conveying anything non-essential had to be kept to an absolute minimum. Otherwise the audience loses the honesty of the experience.
2. Ang Lee seems very intersted in the audience being fully engaged in the emotional experience of the characters, and wants the viewer's to be placed squarely in the middle of the story. For this reason, the actors must never so fully experience and express their emotional nature that nothing is left for the audience to feel. Melodrama can create emotional experience, but not the kind that lingers or lives on. With the very restrained performances in BBM, the audience finished the story and felt the emotions of the characters fully, and far beyond the theater experience.
I think that's why we walked out feeling something, but needing days, weeks or even months to understand it - just as it took Ennis so long to understand what he felt.