In Harvest Moon 64, all villagers have an Affection value, which represents how each villager feels about the player character. This affection is measured by Affection Points, or AP, ranging in value from 0 (dislike or indifference) to 255 (best friend or love interest). While ways to raise a villager's affection are numerous, methods of lowering it are few and almost universally undesirable.
Affection is raised in two primary ways: conversing and gift-giving. In general, each of these methods may be applied once per villager per day, effectively capping the speed at which affection can be raised.
Only one conversation may cause an effect per villager per day. Each villager has a set amount by which each conversation will raise their affection. Most major characters will gain +1 affection each day from conversation; more minor characters such as the Carpenters gain +3 each day, causing their affection towards you to grow much more quickly.
The first time the player character meets a villager, they may gain a special one-time boost to their affection when the character introduces him or herself. After the introduction, they may be spoken to again for the regular daily increase.
During festivals, these values may be different. However, it still applies that only one affection gain may be caused by conversing each day.
Items presented to a character will be taken and will have an effect on that character's affection. Most items will have a positive effect; that is, they will be treated as a gift and raise the character's affection. However some, such as weeds and poisonous mushrooms, will lower affection.
Only one gift may be accepted per villager per day. Each villager likes every item a certain amount, and when that item is presented to them, their affection will increase accordingly. For the large majority of items, the amount will be the same, indicating the base gift increase that determines how much the villager enjoys receiving gifts. Villagers also have favorites that increase their affection more than other gifts. Some major characters have two levels of favorites, gaining a slight bonus for some items and a larger bonus for one item in particular. Remember that only one gift per day will increase affection; additional gifts given will be taken (lost for the player) but cause no affection change.
Negative items behave slightly differently. They will always negatively affect affection, and there is no limit on how many times they can be given per day. Negative items do not count for the purposes of the one-gift-a-day limit; that is, a negative item can be given, followed by a gift, and the gift will cause a full affection change as usual. Each villager dislikes receiving negative items a certain amount, indicating the base gift decrease. For example, Gotz's affection will drop by 5 points if you give him a weed, but Harris is more forgiving, as his affection will only drop by 2 points.
Some objects cannot be given permanently, such as your Dog or your Chickens. These will trigger the once-daily gift change, but the object will remain in your hands. In this way, they function just like regular gifts. There is one notable exception: Karen will not register receiving a gift when shown your dog, but her affection will change just the same. This means her affection can be raised numerous times in a single day by showing her your dog. This is known as the Karen Dog glitch and other names.
Livestock animals and pets also have affection and it is raised in much the same way as villagers.
Your dog's affection determines his performance in races. A higher affection causes higher performance. However, it should be noted that a dog at full affection can still lose a race; there is more luck involved in dog races than in horse races.
Your dog's affection can be raised in three ways, and each raises his affection level by 1 point each day: placing any edible item in his food bowl, whistling for your dog at least once a day, and picking up your dog at least once a day.
Your horse's affection determines his performance in races. A higher affection causes higher performance.
Your horse's affection can be raised in three ways. Brushing your horse once a day will raise his affection by 2 points each day. Mounting and dismounting your horse and whistling for your horse at least once a day will raise his affection by 1 point each day for each action. Note that only an adult horse can be ridden; a young horse gains 1 point for being spoken to at least once a day.
Talking to a sheep (producing a heart above its head) will raise its affection by 1 point. Brushing it will raise affection by 2 points. Shearing an adult sheep with a full coat will raise its affection by 2 points.
The affection of cows determines what quality of milk they will produce. Affection values below 150 produce small milk, below 220 will produce medium milk, and above 220 will produce large milk. If the cow has won the Cow Festival, the cow must have affection above 220 to produce Gold milk.
Talking to a cow (producing a heart above its head) will raise its affection by 1 point. Brushing it will raise affection by 2 points. Milking an adult cow will raise its affection by 2 points.
Cows can enter a happy state, but it is not caused by affection points. However, the day the cow reaches the happy state, its affection will raise by 60 points.