What makes one character unique from another? It has to be more than job, looks, or slang in their speech. In order to really create a truly unique individual with a distinctive 'flavor' all their own you have to get down deep into the emotional heart of them. Too often I see writers make the error of just relying on an external description of their character to carry off personality but really what we look like physically is not predicated by character except in the minor detail. It's more likely to be found in the crow's feet, the way we style (or don't) style our hair, and how we wear our clothes.
I could probably write a whole thesis on the way people dress. There is tendency for those members of society who care about their appearance to suggest that people that are unkempt, dirty, or slobby don't care what people think of them. This couldn't be farther from the truth in most cases. The unkempt is just as much a 'fashion' statement as the well-groomed. Very often the individual is screaming out a political message or maybe just an antisocial "fuck you" at the world. Remember being a teenager and all you could think about was getting laid? There is no such thing as a complete lack of self-awareness in the average human being. Baggy unattractive clothes are often attempts to hide self-perceived flaws from the world: chubby, unfashionable proportions or breasts even.
Many writers will turn to describing 'flashing sparkling green eyes' or other such physical attributes common to romantic thinking, but more important than eye color is where the gaze falls. Do they meet your eyes when they look at you or do they glance away in shyness or discomfort? When we look at the human face there really is no such thing as twinkling eyes or a 'cold' look, yet the entire expression can seem to imply such, but there are a lot more choices out there too: weary, tired, haggard, bags under the eyes, dull eyes, dust on the eyelashes.
I am not suggesting, however, and this is important, that we clutter up our narrative with tons of description. Over describing your character leaves the reader with little to do and still doesn't reveal their true distinctiveness. Description should be used like seasoning - in moderation unless you're making curry. Throw in a comment about the stray few strands of hair in front of the eyes that annoy the observer and don't seem to bother the owner and you're giving us a little taste of that person's character and mood. Mannerisms or nervous ticks can be useful but, again, should not be overused and not everyone has one. We do all have a way of moving that is distinctive. Is your character jerky like a puppet on strings or do they move with the ease of a trained dancer?
I don't necessarily recommend the character sheet or the detailed character biography before you start writing, but if it works for you, by all means, use it as a tool. In my case I just try to imagine my character visually and then as I see the 'play' unfold I 'see' what they are doing and I try and capture the little quirks and visual clues. I like my character to surprise me with what they might do next so I don't care to pin them down with a character biography that is more than just a quick sketch. I can fill in the details as they come to life and they tell me who they are.
How a character performs tasks is much more telling then what they look like. Are they quick and sloppy, or quick and brilliant, slow but methodical, clumsy but inspired? Don't tell us, show us. Does Bobby Schwartz type with two fingers or did he somewhere learn to type? Does he punch the keys emphatically or do his fingers brush softly over the keys? Does he often use the backspace keys to correct his errors? Is he looking at the keyboard or does he stare fixed at the monitor? In the case of Bobby Schwartz, one of my characters, I know he spills a lot of stuff on his keyboard because he eats while programming, and that means he has a box of old sticky keyboards (he doesn't throw anything away) and a few new ones in boxes, or recycled ones stacked on his cluttered shelves so that he doesn't lose time working if the keys start to stick.
In the case of Bobby he is partially inspired by me, partially by programmers I have known, but also he's a mix of other people I have known. I also tend to eat and drink when I'm working at the computer but I have learned not to spill too much on the keyboard, I have a little brush for cleaning the dust bunnies out of it, and I never have a back up keyboard so disaster means I'll have to lose a couple of hours in all probability to go get another keyboard from the local Staples.