When a Character Lacks Social Skills

I'm a people analyzer watcher. Occupational hazard.

As a result, I'm very in tune to when someone is "off." I'm not talking about the people you might see walking down the street talking to themselves, gesticulating wildly at the heavens, and glaring at others like you might try to snatch their spare set of pants tied around their waist.

No, I'm talking about the people who masquerade as "normal." Coworkers who smile and nod as if everything is fine, but secretly harbor demented ideals of world takeover. Clerks who ring up your more intimate purchases at Target more slowly, thoughtfully. Major ick.

Or even people whose little quirks indicate they harbor an as-of-yet-unnamed mental illness. (Lack of official diagnosis doesn't mean it's not there!)

I run into these normal "off" people all the time. Of course in my work, but also at church, in the grocery store, at the mall (most definitely at the mall).

I got to thinking...how can a writer convey a slight "offness" of a character without spelling it out?

Here's some suggestions:

1) Don't use their POV. 

Behaviors are the most descriptive manifestation of mental illness. Behaviors encompass both what a person says, does, does not say, and does not do. Having other characters simply noticing their oddities will clue the reader in.

For example, the POV character could be having a conversation with the "off" character and notice an uncomfortably long silence, as the "off" character has lost the social cue to follow the flow of conversation.

This goes for conveying all mental illnesses, though. If one character has major ADHD, you could have a POV character wonder what he or she keeps looking around at, or how they can't be still while watching the movie, etc.

2) Focus on social mores. 

This is probably one of the biggest clues that someone isn't quite right. Below are a list of pro-social behaviors and a few examples of how a character could exhibit "off" behavior in that arena:

Eye contact: don't make eye contact, make too much eye contact, glare/stare excessively
Personal space: invade too much, stand too far away, are too free with their touch, never touch
Greeting: greet someone too effusively, don't move to shake hands when one is extended, greeting is out of place in the context, don't smile
Compliments: never give them, never accept them, give too many
Conversation: knowing when to speak, when not to, disclosing too much about themselves, not being able to make "small talk," listening/showing interest in the other person, not "tracking" the conversation with smiles, nods, "mmm-hmms"

I think you get the idea.

Let's Analyze:

How else might you convey to a reader that one of your characters is socially "off?"