Traits affect a mercenary’s fighting style or life in the guild. Talents increase the chance of growth in particular stats, and mercenaries can discover hidden talents through combat.
Review a candidate’s traits and talents in "Recruit". For a guild member, open their sheet under "Mercs" and read "Traits and talents" in the "Record" tab.
Compare the benefits and costs of traits when choosing between candidates with similar abilities. For a long-term recruit, look at the stats their talents favor and the role you want them to fill.