Upon meeting NPCs for the first time, the GM may make the PC's make a Reaction Roll, modified by the PRE modifier, as well as any relevant skills (Persuasion, Seduction, etc). A modified roll of 6 or less means a negative reaction, a 7-9 is neutral or indifferent, and 10+ is a positive reaction. A number of things can modify the roll; most such modifiers will range from -2 to +2, with the various modifiers being cumulative:
Outsider - if you are not from the area in question, or are of a different race/species, you usually suffer a -1 penalty. If the two races/cultures have a history of conflict, the penalty is -2.
Religion - people of different faiths usually react to one another with a -1 penalty. If the faiths have a history of hostility toward each other, or the reactant's faith is particularly hostile toward non-believers, it's a -2 penalty. If a religion is dominant in an area, two people of the same faith usually have no modifiers toward each other (as it's pretty much considered the default, after all). However, if the faith's adherents are in the minority of the local area, they have a +1 reaction to each other (the 'we need to stick together' effect). If a faith has been outlawed or is particularly fringe or obscure, meeting another follower gets you a +2 bonus to the reaction roll.
Magic - most people tend to react negatively to those who practice the arcane arts. Those who are known to be spellcasters generally suffer a -1 penalty if the area is not totally devoid of the occasional magician or witch. If in an area where there are no (known) spellcasters, the penalty is -2.
Reputation - If a person is known for being an 'above-and-beyond' kind of person, they will usually get a +1 to their reaction rolls. Also, recent acts of bravery, generosity, etc (standing up against bad odds, buying a round of drinks, etc) will usually get you a (short-term) +1 bonus. Those who have a longstanding reputation for great bravery, selflessness, etc usually get a +2 bonus. Those who are known for being a bit of a jerk, or have recently committed some sort of social faux pas, they suffer a -1 penalty. Those known for being a complete asshole, miser, sadist, etc suffer a -2 penalty. Also, most known criminals also suffer a -2 penalty from law-abiding citizens (if in an area where crime is rampant (i.e. Sanctuary from Thieves' World), the penalty may only be a -1).
Authority - those in a position of local authority (town sheriff, local priest, etc), if they are not known for abusing said authority, usually get a +1 reaction. If their authority extends over a greater area (a prince or high bishop), the bonus is +2.