In a story I am writing an insane character speaks to another. Unfortunately, I don't know how I can convey a person's insanity through their speech. What in their verbal behaviour makes a person appear insane?
The type of mental illness the character is suffering from is important, if you want to be convincing. I agree with @what about doing research at the library on some case studies. But also important is, why are they suffering from this illness - is it genetic? Damage from a head injury? Addiction-induced brain chemistry changes? How does this illness manifest itself? Do they have problems with impulse control, paranoia, their temper, do they see things, hear voices, etc? Given that character's personality, how might that person characteristically respond to their illness? Thinking about it in these terms may help figure out how they should be speaking. Thinking about the character as a real person will help you better render his or her voice.
As many have pointed out, insanity is a very broad term and there are much more specific mental ailments. If this is a major character I'd suggest doing a bit of planning to really determine what their mental illness is and then stay consistent. For example if they have multiple personality disorder, then maybe they're helpful one minute and something cause them to 'snap' and become violent, sort of like a Jeckyl and Hyde character.
On the other hand, if this is just a minor character then you probably don't need to do any research, you just need to make them seem crazy. Suppose the main character asks the insane person a valid question. Some possibilities include:
- Insane person responds to a totally different question in an obsessive level of detail, completely ignoring the original question
- Insane person mutters gibberish that isn't grammatically valid
- Insane person responds to the original question but substitutes their own unique words for common things
- Insane person responds with an answer that is completely wrong or childishly nieve
For example, on that last one, suppose the main character is asking for a lift out of town. The insane person takes him to a rusted-out frame of an automobile from the '60s with weeds growning inside and then hands him a quarter to start it up.
In my own tries to create horror, I used "insanity" as one of the factor to create the proper horror setting:
"What did you do to Pavel?" asked Janette
"Sleeping, hehehe, he is sleeping. Sleeping!"
"I don’t understand what you mean"
"Sleeping! I put him to sleep! Now he is sleeping, hahaha!"
Remember, that most horror stories are really inaccurate about mental illnesses. If you will read about schizophrenia, you will realise that person suffering from that is only perceived as more dangerous. Whereas in horror setting, such person is (almost) always very dangerous to their surroundings.
So, being you, I would not care (that much) about accurate description of mental illness. I would go for perception of such illness and deep beliefs about such illness. Do not forget, that purpose of horror is to scare people. Not to educate them (sadly)
Speech may not be your best option. Action might be a greater tool. The character could speak completely fine (albeit nonsensically in subject), but begin doing some kind of action that no 'sane' person would do.
Your character's illness IS important. A person suffering from one extreme mental illness does not behave like another. Some are aggressive. Some are paranoid. Some are self-destructive. Some won't speak at all.
It might be best to mull the scene over, from an 'end justifies the means' kind of perspective where you just focus on what needs to happen and where you want it to go. Then you take that and use the most effective behavior to get you there, then research that behavior and try to find out how a person who exhibits THAT behavior would speak. Let the story dictate the character in that instance (kind of like a meta-commentary on insanity itself, if you think about it).
Don't focus too much on dialogue with an insane person. Make him do things. Pace back and forth continually, whispering to himself. Laughing, crying, or being downright scary. Make him scream, jump around, get excited over acts of violence, such as murder, or torture. I am a rookie writer at the age of fourteen, and my sister's boyfriend is a horror writer. I get most of my knowledge from him, as he got me into the genre. But don't be afraid to "let him go" a little bit. Let him injure someone very badly, but not kill them. Let him BE lethal. Let him BE destructive. Let him BE who he is. Truly, without a doubt, criminally insane.
I've been told that, when acting the part of a drunk, a good approach is to make it appear as though you are trying to present yourself as sober and failing.
Similarly, the insane person believes himself (or herself) to be sane, and is often desperate to prove that fact to others. Or, if not aware of how other view him, he may likely think of others as insane. In my own experience of real people with mental issues, the surest tip-off is their endless complaints about others.
First figure out why you need this character to be insane. What insane act or decision must they make in order to advance your plot?
Insanity generally involves a failing model of the world and people in it. Like truly believing if you hold a white feather, you will be able to fly if you leap off a building. A man truly believing his dog talks to him and tells him certain people are immoral and evil and must be killed. A man truly believing the voice in his head is God, granting him permission to kidnap a fourteen year old girl and make her his wife. Or a man truly believing through fervent prayer that God told him to murder a doctor for performing abortions.
These are all failed ideas about how the real world works. A person is NOT insane unless they have such a mental model that will put them in danger or other people in danger. Their reasoning is broken because they believe things that are just not true, about other people, about God, about physics, about magic or the supernatural or the state of the world.
Truth can be relative, of course. Four centuries ago is was common to believe that a magical object existed that could turn common things into gold. No such thing exists, but it was not insane to believe it could exist, or even go in search of it. Today, we might classify such beliefs and actions as insane.
That said, for modern characters, the way to portray insanity is best done through action (as other answers say) but for dialogue the best way is to develop exactly what the rules are of this characters failed model of how things (and people) work and behave and believe, then have their reasons for doing things depend upon that failed model. You can reveal their "crazy" beliefs in dialogue as the justification for doing what they are doing.
Their dialogue doesn't have to necessarily be distracted, or frantic, or anything of the sort. It can be like talking to a professor. It is their bad reasoning from bad principles that betrays them as insane.