Nah, most of my characters are already insane to begin with.
Anyone who isn't insane will have seen it before. Cubed. It just isn't happening these days. :/
Reference Evil Dead 2 for proof of modern people going insane.
I think characters in stories have tolerance levels - it all depends on whether they breach that level and 'snap'.
The easy way to begin writing for my characters is by relating some horror in recurring dreams that caused them to instantly 'snap'.
The harder way for my characters is by tolerating slowly spiraling affronts to their sensibilities that they have to manage somehow before they've had enough to finally 'snap'.
Don't know if this makes sense as I don't know your writing style but hope it gives a couple of ideas for your character's development in your stories.
Think about your character and write in accord with the way that seems natural for their reactions to their situations.
Writing should be a pleasure, not a chore. 
*shakes head*
Really, there's nothing new under the sun... while an Ancient Roman, if exposed to modern technology, would probably start randomly worshipping people or simply go mad... there are things that our hypothetical ancient Roman would be totally unbothered by (Gladitorial games, hand-to-hand combat that mostly involved sorta-kinda-sharp lumps of metal, butchering your own meat, open slavery, even human sacrifice to a certain degree) that would violently disturb a modern viewer.
While the specifically Lovecraftian horrors might not have as great an impact on a modern observer, there are other things that would unhinge us just as easily. Human nature really doesn't change all that much.
"A soft answer turneth away wrath. Once wrath is looking the other way, shoot it in the head." Seven Habits of Highly Effective Pirates
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The things that will always cause the human mind to break will be the ones that the human mind literally cannot comprehend. Buildings stretching off into angles that make your head hurt just looking at them, things like that. Until we can find someway to understand in more than four dimensions (including time) then anything that operates on any additional dimensions is really going to mess with our heads.
Statesman said let there be heroes, and there were heroes.
Lord Recluse said let there be villains, and there were villains.
NCsoft said let there be nothing, and there was nothing.
But would it mess with them enough to make us snap? There are already plenty of pieces of art that can replicate such effects. For example, a perpetual waterfall where the water falling off the edge goes back up to fall again up a stream using a straight level path. Sure, such things are hard to comprehend or even look at, but would they really drive us insane?
Actually, no. Interestingly enough, nature (or your respective deity of creation, if you like) seems to have had some forethought there and installed certain safeguards within humans.
I'm not going to go into all the detail here, but all in all it boils down to the normal senses of a human being incapable of perceiving anything higher than a 3-dimensional world. This is, for lack of a better term, why humans cannot 'see' time, but instead only 3-D snippets of its sum.
So actually, if you guys were to look at a quintadim object, for instance, you actually couldn't perceive it with your standard senses, as they are not properly 'calibrated', if you will. Instead, your mind will come up with something to put in its place, such as random colors maybe - I'm not entirely sure, as it's suggested that this varies from person to person.
However, the average human will not go insane from looking at an object of a higher dimension. In fact, chances are you're doing it right now, but don't even realize it because you lack the senses necessary to properly perceive said object.
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
I'll go with creation.
*Pops open skull cap and scratches brain*
Yep.
Say, does anybody have a nail-gun lying around that they don't need...?
Well, all I can say to that is to take a look at the Deathstalker series by Somethingorother Green. He frequently has the protagonists fight creatures or things that defy rational explantion and makes special note that any normal person would have their mind blown at the simplest sight of such a thing.
Green does lots of crazy [censored] in those novels. Like a kid with the power to render 2/3rds of the galaxy lifeless with but a thought or a psychic abomination composed of the brains of thousands of people with psychic talent that with it's mere prescense can cause an entire planet to go insane.
Crazy [censored], man.
Statesman said let there be heroes, and there were heroes.
Lord Recluse said let there be villains, and there were villains.
NCsoft said let there be nothing, and there was nothing.
Oh, I'm not saying that the sight of something really horrid can't make a human go insane. There are a great many things that can do that. Nor do I contest that someone flooding a human mind with insane thoughts won't make it go insane. My post focused solely on things of a multidimensional nature causing insanity by perceiving them.
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
Would it be possible for the safeguard that keeps us from going insane when looking upon something we cannot comprehend to get overloaded?
Cause if so, that would suck.
Statesman said let there be heroes, and there were heroes.
Lord Recluse said let there be villains, and there were villains.
NCsoft said let there be nothing, and there was nothing.
Not really, it's a very versatile defense. It hardly matters HOW complex the painting is, a simple bucket filled with white-out can do wonders...
Depends on what that safeguard is.
I usually use "I see this. This is happening. Time to deal with it."
My Stories
Look at that. A full-grown woman pulling off pigtails. Her crazy is off the charts.
No. It's like a blind person not being able to see. You'd have to make his eyes work first. 
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
My original question was geared more toward the passive effects of the Chulhu Mythoes; i.e. looking upon R'lyeh, observing a Great Old One without them noticing you, direct interaction with one of their minor minions that do not have mental abilities. I have no doubt that were any human to draw the direct attention of one of the Elders that would be the end of it. However I personally can not think of anything that terrifies me or makes my brain hurt and am therefore having difficulty conveying that effect onto my characters that should be experiencing those feelings and emotions.
I grew up on a farm. We did our own vet work and butchering. I was raised to see life as life and no particular form of it being any better than another. This stood me in good stead when I later became a Paramedic as I could easily deal with what some people do to themselves and others. I understand there are insane people but these people have physical or chemical problems that are keeping their brains from working properly and were not flipped out by something they saw. Was the effect Lovecraft trying to convey similar to what we now term as Post traumatic Stress Syndrome ? Somehow I think he was going for something on a greater magnitude, more visceral. Does the huge amount of information and images we now have access to via the internet benumb us to the effects of these other worldly situations (presuming we are able to even perceive them) ?
Perhaps I am already insane by Lovecraft's standards and that is why I am having this problem. I have several of H.R. Giger's works and they do not bother me in the least. If this is the case then it is mealy a definition problem (What is insanity) and Lovecraft was a pansy.
Giger's work NEVER bothered me. I've seen plenty worse in my nightmares.
H.P. was a horror author. Probably had a dim view of humanity, too. His work was meant to affect people with a more limited scope of reality, especially in a time when people were horrified to find women wearing pants.
I usually refer to Lovecraft as "His Era's Steven King." I think the worst effect of his work is the insanity of trying to pronounce the names of his characters properly.
Is it Kuh-Thooloo? Ca-Thooloo?
Damn headaches...
My Stories
Look at that. A full-grown woman pulling off pigtails. Her crazy is off the charts.
However you say his name, I know what Cthulhu's favorite ice cream is...
Fhtagn-Dazs
Global - @El D
Servers - Protector
I've seen plenty of crazy [censored] in my life. My thoughts at the time usually run along the lines of "Screw it let someone smarter figure it out" and continue doing what I gotta do. Going on wreck calls with the fire department when you're 6 is usually one of those things that psychiatrist's believe lead up to insanity. But hey look at me I'm perfectly sane.
.
.
.
Even though I'm usually told I was born in the wrong century.
I do not know my reasons for posting, but post I must. ~~Build
"We can't stop here, this is bat country."
--Johnny Depp
"I ain't bi-curious, I'm a man. Why do you think I'm wearing this tight spandex and got all these muscles?" Meatwad
Some people call me insane...I tend to agree with them just so they will shut up, and /or leave me alone.
Though I dont really think I'm insane...at least not in the way that certain pictures and stories sometimes make me...<_<
"You, dear sir, are a legend. "- nelly. "PIE DOESN'T HAVE TENTACLES!"
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I don't remember who said it but it went something like this "Only crazy people say they're sane."
Looks like me and Aegis are in the same boat. Just agree, nod, and at the first chance you get. . . . AS!
I do not know my reasons for posting, but post I must. ~~Build
"We can't stop here, this is bat country."
--Johnny Depp
"I ain't bi-curious, I'm a man. Why do you think I'm wearing this tight spandex and got all these muscles?" Meatwad
I am legally insane. I have a psychoththerapist and 'special' medication that I am required to take, and I have someone who comes over every week to check and make sure that I have taken it.
And I will say this. IF I really am insane, I wouldn't want to be sane for a minute. Everything is so pretty. Like crystals...
Bah, there I go again. Ignore this post.
Wow...
Just...wow...
I don't even know how to respond to that.
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
Awww that's not you Devious. You've made me sad. 
Say something neat like you always do. What's wrong...?
Well for one thing, I happen to be legally insane. And yes, I am quite serious.
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
In the Chulhu stories and games, the characters are often driven mad by what they witness. Alien cities, unspeakable grotesqueness, unholy rites, they all unhinge the minds of the observers. But consider the age and culture in which these works were produced. Several concepts and ideas we accept today would have been outrageously ludicrous in those times.
So my question to you my fellow gamers, would a modern day protagonist presented with the same scenes be better able to withstand the unnatural assault upon their senses ? Or would they just as easily start with the gibbering frothing ?
ps - this may be in the wrong place but as it sorta deals with character development and character backstory I thought I would try here first.