Half orcs, the great barbarian race, the mean green fighting machines. Ugly as sin and as powerful as a runaway bulldozer, they are popular with everybody who wants to play a tough, rough and gruff pounder. Rarely employed for anything other than to punch and rend, half orcs are usually played as brutal thugs. But what distinguishes a half orc from a common human bruiser? What does that half orc component do to their mind? How do you play a half orc right, and not just a green faced human?
Yes, half orcs are large and tough and powerful. But so are large, tough powerful humans, so that's not enough for your player to base their rp on. Let's dig deeper. What is an orc like, and what part of that mentality (or lack thereof) might the half orc have inherited? Orcs are cruel, domineering creatures driven as much by instinct and cunning as any kind of rational mind. Sadistic, elemental, cowardly, they respect force and power, and take advantage of weakness and those below them.
Now, a half orc won't be all straight up orc; he's got as much human in him as orc, and so you have to balance that out. However, there will still be elements of that mindset within him, so think it through. First, there's an instinctive drive to dominate. To control those you can get away with pushing around, and to seek to overthrow those above you while doing as they wish. It's hard for a half orc to understand basic concepts such as democracy, fairness, justice. The abstract is hard to grasp; much more real is the sword, the fist, the ability to grind one person under your heel.
Second, that desire to dominate is often coupled with cruelty, with sadism. The orcish heritage doesn't make them want power so as to govern fairly, but rather to aggrandize their egos and glory in their power. Those beneath them will be tormented, pushed, punished and abused. While a half orc won't go to that extreme, it's a rare half orc that treats servants, shop keepers and anybody else who is clearly in a position of serving with anything like kindness or respect.
Orcs think in terms of the now and today. They have trouble planning for the far future, which is why they live nomadic lifestyles, raiding and preying on others, never building orcish empires of their own. Those same trends will influence their PC; they don't look too far into the future, nor too far into the back. Reflection and philosophy are not for them. Rather, they are more liable to want to take the direct approach, kick in the door and bang heads. They spend what they earn, they enjoy life to the maximum each night, only to stumble back out into the street broke and looking for work the next morning. It's all about the here and now, living life for physical pleasures, and leaving the abstract and unsatisfying for the refined foolishness of elves and humans.