Perfect-Organism says Hicks was quiet and reserved, but that in no way says he was a pacifist. I would also say quiet and reserved (at the start) , but also reluctant, as in, reluctant for leadership or responsibility. That bit where Ripley points out to Burke that Hicks is the successor in the chain of command, and Hicks's shoulders slump and he sighs so heavily - priceless!
I've always imagined him as coasting through his time, lack of ambition holding him back from further promotion. Ricco Ross says after 2weeks of James Remar he could barely recognise Hicks in what Biehn was doing with the character. Remar was forthright and dominant; Biehn was low key and softly spoken, and it took Ross a while to understand Biehn's acting choices and overall interpretation. I think we lucked out there.