The basic personality of a character can be found by rolling a rising sign, a sun sign and a moon sign. If you're feeling Freudian, you can think of them as the ego, the superego, and the id. Astrologically, however, the rising sign is the image a character projects; the sun sign is her (or his) sense of identity, her rational goals; and the moon sign is his (or her) unconscious urges, his unknown resources.
These three signs form the core personality. Additional planetary influences can modify the personality, but not change it radically.
In medieval times, only seven planets were known. When rolling a horoscope for a character, it is best to use the medieval system. Not only is this usually more authentic, depending on the gaming system you are using, but it is less complicated as well. Complex characters are all very well and good, but there's such a thing as going too far. Also, since the outer planets move so slowly, age cohorts all have them in the same sign. This lends a commonality to age groups that would be missing if all characters rolled their outer planets individually.
Inner Planets: MERCURY and VENUS are not free be in any sign. Since their orbits are inside of earth's, they can never be far from the sign that the sun is in. As a rule of thumb, Mercury can only move within one sign to either side of the sun sign and Venus can only move within two. That means that Mercury can only be in one of three signs, once the sun sign is known, and Venus con only be in one of five.
MERCURY is the planet of communication and intellectual ability. Mercury's sign indicates the sort of thing that the character is likely to talk about. Recurring conversational gambits begin here. It may also affect a character’s voice or style of speaking.
VENUS is the planet of comfort. The sort of things that the character finds familiar or beautiful are determined by the sign that Venus is in. Purchases, especially of clothes or homes, may be influenced by Venus.
MARS is the planet of action. It is easy to find the energy to begin to do the things that pertain to the sign of Mars. If there is no support for the activity elsewhere in the chart, though, it will be difficult to find the motivation necessary to finish these activities. Many impulsive acts begin here.
JUPITER is the grand benefic. It influences the character's religion and generosity. It may indicate the sort of things that the character does well and easily.
SATURN brings limits. This is the area where the character will struggle and make mistakes. It is a recurring lesson. Recurring nightmares may begin here, if the character is subject to portents.
SIGNIFICANCE OF SIGNS:
Now roll up a sign for each planet and assign the traits to the categories. Here are some of the personality traits traditionally associated with the signs of the zodiac. If you don't like mine, try checking out a book from the library or making up your own.
ARIES: Enthusiastic and assertive. Good at beginning, but impatient. Self-assured and blunt. An independent, ardent competitor.
TAURUS: Persistent and hard-working, but stubborn and slow to change an opinion. Comfort seeking, rather than ambitious. Cautious but courageous. Not talkative - calm and sure.
GEMINI: Restless, tending to chatter, gossip, and fidget. Cool and decisive in a crisis. Charming, but uninvolved.
CANCER: Protective/possessive of people, places, and possessions. Would rather stay at home, but will make sacrifices for friends and family. Moody - fears ridicule. Some cancers fall apart over little things but handle true crises with calm.
LEO: Proud, outgoing, dramatic, and self-centered. Wants all of the attention. May sulk if ignored. May cause a crisis just to liven things up and get a chance to take the spotlight.
VIRGO: Seeks purity, perfection, and wholeness. Urges others to seek same. Can turn into a nag. Disciplined and over-organized. Sometimes misses the forest for the trees. Sometimes uses sardonic wit for the "betterment" of others.
LIBRA: Amiable, peace loving. A good arbitrator if uninvolved, but a little wishy-washy otherwise. Can see seventeen sides to any argument and sometimes has trouble deciding things. Seeks harmony but can get fussy.
SCORPIO: Passionately intense but intensely controlled. Can be cruelly blunt. May have a brooding, self-destructive side. Can look at you as if he knows exactly how you'd cook down in a compost pile. Likes to take things apart. A seeking intellect, uninterested in surface appearances.
SAGITTARIUS: A cheerful, enthusiastic extrovert. Generous and helpful but oblivious to social nuances. Can be blithely blunt. Idealistic, energetic, and often inspired to quests.
CAPRICORN: Industrious, unobtrusive, and ambitious. Can be bossy or strait-laced. Understated sense of humor. Determined to rise slowly to the top. Any wild risks must be analyzed first and found to have a good probability of advancement.
AQUARIUS: Adaptable, inventive, and gregarious. Creative and rational, intuitive and eccentric, may be rebellious when confronted with rules. May be devoted to, but detached from a group.
PISCES: Secretive and nebulous. May be forgetful and absentminded. May be blissfully mystical. Decides by the feel of things rather than by analysis. Sometimes vain. Occasionally treacherous.
[I keep saying that I might post the planet/sign/character trait table and I keep finding stuff that should possibly be posted first. This set of instructions can either be used instead of the table or in addition to the table.]