Your in-character actions should reflect your chosen ability scores. For example, any intelligence score lower than 9 means you cannot speak properly (we’ve seen a lot of eloquent half-orcs with int scores of 8 or lower).
Another fine example: a low charisma score would likely make you uninspiring, a poor leader, less concerned with personal hygiene, and simply not as likeable as someone with a higher charisma score.
Please try to reflect your character’s ability scores through your role-playing as best as possible.
Your in-character actions should generally reflect your alignment, if they do not, a DM can and will adjust your alignment accordingly. A change in alignment may have serious in-character consequences for many classes (see paladins and druids for instance).
In any case, alignment is an important factor in determining your character’s actions as well as her general philosophy.
The various races will behave uniquely. You are encouraged to research typical racial behavior, and to play your character accordingly.
The Forgotten Realms is a world populated by many different races, and is rife with racial tension. Many races have hundreds if not thousands of years of enmity between them. It is well known that all elves and dwarves despise those of orcish (thus half-orcish) descent. In fact, most races react negatively toward half-orcs.
Keep in mind that someone acting with prejudice toward your character’s race is role-playing the part, and that it is not in any way a personal insult. We in no way condone real life racism (and would boot a player for using real life racial slurs), but this is not real life, it’s only a game.