[RPG] How to play a dumb and un-charismatic character

A character with a High Intelligence but Low Wisdom might be incredibly book smart, but continually makes poor decisions, is absent-minded in the extreme, and tends to miss "little picture" stuff in favor of "big picture" stuff. This is the incredibly learned wizard who basically needs a handler wherever he goes due to his eccentricity. One example might be Walter Bishop from the TV show Fringe, if that makes sense.

A character with Low Intelligence but High Wisdom might be considered a dullard by society's standards, but has some matter of insight, or might be very attuned to the smaller things in life. This person might be illiterate or might be an idiot savant, but they have a way of picking up on the simple, straightforward solutions that other people miss, perhaps because they're going for the "big picture" stuff.

Those are just two popular ways to interpret the disparity in those two stats. I'm sure you could come up with other examples as well.

The successful dumb characters I've seen aren't just dumb. Develop the character as much as you would a character that's not supposed to be dumb.

I find that characters who use one syllable words or weirdly bad grammar but seem otherwise unimpaired get boring pretty fast. Your character can often follow his friend's lead most of the time, but when he is left on his own, he makes mistakes, or does the obvious in combat. If his friends told him to do something and the situation changed, have him continue to do what they told him to do because he doesn't understand why he's doing it in the first place.

Also, don't overdo it. You probably rely on others for figuring certain things out, but you can remember how people have treated you. If people are condescending to you, you can tell unless they are subtle about it. When it becomes obvious you've done something stupid, you get ashamed, or angry, or are embarrassed and don't want anyone to mention it. You don't use complex sentence structure, but you usually speak normally enough that it isn't immediately obvious that you are dumb.

This can be fun for the table because it's kind of a problem solving exercise to give your character instructions that cover enough possibilities but aren't too complicated to understand. Some problem solving and strategy things are kind of out for you, but you can spend that time figuring out what your character's understanding of the situation is. To a point - you might want to ask everyone's help in not having the plot be a series of riddles or puzzles, unless they specifically give you something to do (e.g. your character is in a riddling contest matched up against a talking chicken "What am I sitting on?" "An egg". "What do I like for breakfast?" "An egg." "Where do chickens come from?" "An egg". "What came first before a chicken?"...).

For a really dumb character role playing them realistically would probably not be fun, since they mostly have no idea what is going on - it would be like trying to role play a dog. If it's a comic campaign where you are as dumb as would be funniest at that moment it might work OK, but it probably gets old within a few sessions at most. Or not. If you are having fun don't let me stop you - I'm just thinking about other people's characters that have worked out for my taste.