Q:

Fleshing out a background and roleplaying it

I am a big fan of fleshed out, rich, interesting backgrounds, that give the feeling that the character is real. For that reason, I really like to give characters traits that everyday people have, seeing glasses, a smoking/drinking habbit, some disorder or some heavy mental strain inflicted in the character's past, a mania or a certain liking etc.
It is somehow hard to tailor such ideas with more general ones about the character's story and compose a good, well written background with a distinct feel. Aside from reading books to get a better understanding of how one portrays a character reallistically,

What are some "guidelines" on how to flesh out a character's background, and the character himself? 
How can one tastefully roleplay the character's traits -especially mental- that are delicate and mostly sublime?

A:

Back in the day, and today available as a PDF on the web somewhere, is Paul Jaquays' Central Casting: Heroes of Legend.

A detailed, stand-alone system of tables, charts, and role-play guidelines for creating thoroughly individualized role-play characters. By following simple, step-by-step instructions, both game masters and players can give life to player and non player alike!

I highly recommend it when you just don't want to conjure something anew, or just need hints to spark the conjuring fires. My most recent Pathfinder ranger was built using HoL and I had a lot of fun using the book. It even has charts for developing flaws based on your attributes/characteristics (or even using the charts to instead "develop" attributes from events in the characters life).
More recently, Masks: 1000 Memorable NPCs for Any Roleplaying Game from Engine Publishing is now available. There are three genres covered: fantasy, modern, and sci-fi. While ostensibly a NPC tome, the authors have crafted a valuable book for getting ideas for memorable characters as well.

A:

It will depend on your players largely, a lot of the time they will have trouble answering simple character questions derived from  "who are you" or "what do you want".  one of them will write you pages and pages of "backstory" that tells you as little as the "unacceptable" answers to who are you while another will scrawl a few lines on a napkin at taco bell that not only tells you who they are, but goes beyond to tell you how they think on top of who they are.  Just as "who are you" type questions are difficult for some players, "what do you want", "where are you from", "where are you going,  "who do you serve", "who do you trust", "why are you here", "do you have anything worth living for", and "why are you here".  Some players will give you everything that you need, others will give you pages of nothing and wonder why you never involve anything from their backstory even though you mine the hell out of that other dude's taco bell napkin.
Instead of a backstory, interview the character with questions like these ones copied out of a pdf of a book about gamemastering available here as they will cover .  Do not accept "I don't know" and do not phrase anything that allows for a yes/no answer as those answers are "unacceptable" just as the ones in the video clip above were. "ok why" in response to these types of questions will make excellent pointers for excellent background stories as well.  Dig until the character is solidly defined

What do you hate most in the world
What would you like to change in the world
What do you love the most
What kinds of friends do you have
How do you feel about authority/military/police/government/the church/etc

There are a lot more good ones there broken down into different character aspects like their own personal morality (D&D style alignments are epic freaking fail, ignore them since Dexter Morgan is paladin style LG by RAW Even within the strict interpretation of D&D alignments, I've seen a LE rogue selfishly use bluff & diplomacy to convince a LG paladin to run from the authorities rather than risk fighting them trying to explain why there was a dead body in the room to keep the paladin from being at risk of falling & continually trying to keep him from falling simply because his pristine image traveling with him allowed him to get away with more& bigger LE stuff while the paladin continued to allow it because the LE rogue was careful not to go too far & did more good than bad in order to keep from possibly losing his moral high ground bluff card), goals, personality, etc.  
Instead of just asking for "a backstory" that will oftewn come back worse than useless, ask a bunch of questions to get inside their heads.  After things are fleshed out, they will no longer have a sheet of numbers, you can even give them free stuff to help it be more real.  Let them write backstories that give detail about some of the questions if they want, but don't push them or you are back at square one with pages and pages about nothing of value.

