StrengthA character with low Strength lacks muscle mass. He receives penalties to hit in melee combat and penalties to damage with most weapons. He can't climb well, lift much weight or exert much force. While this might mean a character without athletic traning, there are other options.
Weight factors into Strength - the larger you are, the greater your potential. For Moderately low scores (6 to 9) you might be large without muscle tone: you can throw that weight around, but you are weak for your size. For lower scores you might be emaciated: an apparition with sunken cheeks and fleshles limbs.
Age is another explanation. A low-Strength character could be old and frail, which works well for spellcasters, as age gives appearance of experience and knowledge. Perhaps a character is particulary young. Playing a youngster has it's advantages: adults ignore or underestimate childrens, and a child could be more flexible and easily hidden, but you should avoid life skills such as Knowledge, Profession or Craft. Regardless, experience translates into maturity, and with maturity could come Strength.
You might have personality quirks centered around your meager muscles. Since you can't participate in physical labor, you might see yourself about it, assuming every powerful frame holds a weak mind. You might take a "those who can't do, teach" attitude and direct the labor of others, or you might be envious of or deferential to those stronger than yourself. Even this most physical of abilities offers plenty of opportunities for roleplaying.
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Dexterity
A low-Dexterity character suffers penalties for ranged attacks, Armor Class, Reflex saves and any skills requiring coordination or agility. You're useless at moving quietly, can't mantain balance, neither throw nor catch well and routinely dodge right into attacks. "She's clumsy" is the first response to a low Dexterity, but several other possibilities exist.
A handicap might be the cause of a low Dexterity, such as the classic peg-leg of a pirate. Perhaps you recently lost an eye: the loss of depth perception and peripehral vision could explain a low Dexterity. Equilbrium-altering wounds could be thereason. Perhaps your wizard was too near the origin of a Shout spell, or maybe your priest heard her god's true voice.
Alternatively, you could have tremors and poor motor control: you could be a wizard whose studies of the unknown left her with twitches she can neither stop not explain, or a veteran whose constant drinking has dulled the memory of was but leaves her unsteady. Subterranean races often risk cave-ins, and many walk away with injuried limbs and frayed nerves. Such people might be clumsy, but they are hardly inept, and their conditions can at least lend them a haunting history.
Roleplaying a low Dexterity depends on the score's rationale. Those hindered by a physical problem might use it to avoid hard or dangerous labor, or they might refuse assistance to prove they're as good as anyone else. You might be perpetually apologetic or constantly upset over your blunders. Alternatively, you might make jokes about your apparent ineptitude or develop excuses to cover for it ("Somebody keeps moving that plant!"). The key to roleplaying a low Dexterity lies in how the character feels about her fumbling and how she expresses it.
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Constitution
Low-Constitution characters have penalties on hit points and Fortitude saves, as well as Concentration checks when casting spells. They are easily knocked out and easily killed, since having poor hitpoints is a fatal flaw for an adventurer.
Many of the physical descriptions of a low Strength or Dexterity can translate well for a low Constitution. Characters who are extremely underweight or overweight might be easily winded or unable to resist injury. An eldery character's health could be ravaged by age. You could have old wounds (such as an arrow through the lung) or be a plague survivor. Old wounds explain a lack of stamina while emphasizing your will to survive.
More dramatically, you might be the victim of a long-term disease. For example, tuberculosis can take years to kill, leaving victims weak and coughing up blood, but Doc Holliday was the most feared gunman in the West even as tuberculosis was killing him. In fantasy settings, even more insidious illnesses exist, sapping your life while allowing limited activity. A character facing his own death head on and refusing to wait for it engenders respect.
A low Constitution often shows itself in personal behaviours. You might constantly push yourself too far and then collapse until forced to move again, or lean against the wall and rest at every opportunity. You might have remedies or medicines you constantly prepare to bolster your Strength, or obstinately ignore your problems. Roleplaying a low Constituion relies more on what you're doing and how you're doing it than what you say.
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Intelligence
A character with a low Intelligence learns slowly and reasons poorly. She has penalties on the number of skill points per level and on any skills based on learning.
Some thought should be applied to why you have such a low Intelligence. You might have never had an opportunity to study, or perhaps you were forcibly banned from doing so. You could've grown up on the streets, among war refugees, or as a slave, where any attempt at learning was severely punished. You might have been raised by animals, a fantasy classic for characters with a nature bent. While brilliant people might come from such backgrounds, most survive by intuition, guile, perception, and will - in other words, traits associated with Wisdom and Charisma, not Intelligence.
For a great challenge, you might be an amnesiac, forgetting anything more than 24 hours old. Head trauma occasionally causes such problems. You are reborn every morning, with no guide save those around you and whatever notes you left for yourself. You may need solid roleplaying skills before trying something this extreme.
When playing a low-Intelligence character you could play it for laughs, as with the barbarian who claims "newly discovered" towns and names them after her father. Or you could play it for terror by taking offense at people talking down to you and snapping into vidictive rages. You might insist on having a say in any discussions and want your not so helpful ideas respected, or you might keep your mouth shut to avoid looking the fool. You could repeat others' suggestions as your own or constantly request more time to work through problems. Remember that a character with a low Intelligence can learn and reason through everyday things, they simply can't do it quickly or reliably. You needn't stiffle your every creative thought, as long as you aren't constantly brilliant or quick.
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Wisdom
Someone with a low Wisdom lacks common sense, perception, intuition and willpower. He is penalized on both his will saves and skills involving untrained experience or awareness.
A poor Wisdom score is the hardest to explain, since it covers such a broad area. Wisdom is something one gains with age, so you might consider a young character to explain a low Wisdom. Take care with this, since a well-played low-Wisdom child will quickly get himself into trouble and might drag the party down with him. Perhaps you come from a culture vastly different than the local norm: detail this culture with your DM, with both good and bad points, and stick with it. You'll miss local cultural cues, make assumptions based on false premises, and argue against things the rest of the party takes for granted ("Letting women drink ale instead of wine ? It will corrupt their souls!"). You wouldn't be stupid, merely ignorant of the local custom.
When playing a low-Wisdom character, make it clear that the party can't rely on you in the long term, In the short term, during the course of a fight or while exploring a dungeon, they can keep an eye on you and trust you will do your part. But if left on watch alone you might fall asleep, get engrossed in spell research, or otherwise lose focus. In delicate negotiations you'll let secrets slip or take offense where wiser diplomats would not. You might forget to mention things you've learned until a critical moment, or assume that your companions must know things, and thus never mention them. The party should probably put another PC to watch over you, just to make sure your impulsiveness doesn't endanger them.
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Charisma
A low Charisma comes from a lack of faith in yourself, translating directly into an inability to interact with people. The character is penalized on all skills that involve social interaction, as well as any non-skill interaction checks.
When dealing with a low Charisma you should remember that these personality traits don't appear without cause - no one is born with a 3 Charisma. What caused you to lose faith in yourself ? Possibly your family or culture valued everything you were poor at and never praised your skills: shy to an extreme, you avoid conversation for fear of being insulted, belittle your abilities, and view any comment as criticism. You might be an escaped slave: deeply ashamed of your past, you either lash out verbally or meekly follow suggestions as orders. You might have once had personal faith, now shattered by some terrible event. This trauma need not have been your fault - you believe if to be so even if the world holds you blameless. An accidental killing, a failure of nerve on the battlefield, the inability to save a loved one, or failling a test of character can shake your self respect. Once that is lost, it is hard to gain the respect of others.
Alternatively, you might be extremely ugly. Not just homely, but malformed to the point where children scream and woman faint. faced with such reactions you see yourself as an outcast while still desperately wanting to be accepted by society. Note that this is very different from the high-Charisma half-orc who looks like ten miles of bad road but whose magnetism unites an army - appearance matters to Charisma only if you let it matter.
When roleplaying a poor-Charisma character, keep in mind why the character has such a low score and run with it. This might mean never vocing a suggestion (even if you know what to do), or voicing every single suggestion loudly and authoritatively - since if it isn't accepted it damages your already fragile sense of self. Belittle NPCs to prove you're better than them, and even make jibes at other PCs' expense (though not too often, or you'll reduce other people's enjoyment of the game). Here's another idea: start whispering ideas in character to another player, letting him voice them to the group - since your character thinks no one listens to her ideas, she can contribute without putting her ego at risk.
This was taken from another NWN Ravenloft focused server by DM Knas.
I believed it was very informative in giving guidance to what is the roleplay expected from low stats or stats in general.