Differentiating characters

Conflict. It's not like they all should get along, and even the smallest amount of conflict between the characters can be inflamed into a great foil.
You can kindle character conflict in many different ways, but I find plot (as in, the characters clash because their goals/motivations intersect) and personality conflicts to be fairly easy.

Characterization. And that's not how they dress, how they speak or hobbies or education level.

I have a group of friends from university we are all English graduates, but we are far from being clones of each other. We have different accents (I'm from England, Alison is from Ireland, and others are from different areas of Scotland, with different accents.) We have different politics. We have different personalities (I'm far more driven/ambitious), Tracy doesn't hold anything back and lets rip (even at superiors), Edwina is nervous and lacks confidence), and Alison is a bit of a mediator.

Think character traits, and how your own friends differentiate.

^+1.
Treat every character as an individual, and you will have less problems in keeping each of them unique and different.
They probably had different childhoods, childhood experiences (parents together or divorced? Where did they used to go on holiday? Do they have brothers/sisters? Etc) that have shaped them and their view of the world, the way they behave, etc. Cultural background can still be very varied if you dig a bit deeper.
Of course there is much more, but just as an example of a starting point to differentiate them.

Just my opinion, of course, hope it helps.

Take your own circle of friends as an example. Like is drawn to like, yet each person has their own unique blend of characteristics. One person may have a dry wit, his friend may have no wit at all. Another person may face a difficult situation with braveness, while her friend acts with cowardice.
Then there's the character voice; the individualistic speech patterns, choices of words, size of vocabulary. Body movements, facial expressions, reactions.
These elements are the fundaments of individuality. You've mentioned other elements that further differentiate people. There are also such things as religious belief, inquisitiveness, passions such as poetry or art, skills, age, personal history, cultural history and beliefs.
In short, every tiny thing about each person contributes to his or her overall picture and overall voice, and should serve to make each character in your story distinguishable from all others. This should be true of all main, major and cameo characters, plus most minor characters. If you're finding two or more characters seeming to be identical (or as near as damn it) then one of two things is true:
1.) Your characters are not yet fleshed out enough, and need more 'characterisation' and 'voice'.
or
2.) You have unnecessary characters / You might want to consider combining two or more characters into one.

Fantasy faction recently had an article about this, though it's more about how to use narrative to immerse your reader in the character. The author did talk some about the different things a character will notice, as well as the words used in the associated narrative, will also establishes their voice and personality.

Thinking about a character's values can be useful here. Two people from the same family can be very different in some ways. Even little things can find their way into the narrative, even for the little things. Someone who loves sunshine and salt water will experience a trip to the beach very differently from someone who prefers to be inside playing video games.

She put her towel down on a golden patch of sand and inhaled the moist, salty air. Finally some time to relax. "Last one in is a rotten egg!" She sprinted towards the water and threw herself into its cool, tingling embrace.

vs

She put towel on a dry, crumbly patch of beach and eased herself down. Great. The sand was already sticking to her skin, and she hadn't even put her lotion on yet. Breathing shallowly against the stinks of seaweed and dead fish, she pulled her ipad from her bag.

Quirks, both in personality and appearance. Even if characters have the same economic status and background on the surface, they are as unique as real people. Someone's a shopaholic, or has a disabling fear of flying, or is a dwarf, or is covered in tattoos and piercings, or has OCD.

Think of the people you know who seem similar to you. How are they different from you? What are the everyday quirks that make them not you? Now find a way to work these quirks into characters in a way that doesn't seem obvious but that allows a reader to distinguish them from one another by how they move or speak.

HTH and GL.

To balance out the other posts here, I feel obliged to warn you away from turning your characters into enormous bundles of unique quirks. It can be really tempting, but the results aren't pretty. Beyond that, take all these guy's advice.

StormChord said:

To balance out the other posts here, I feel obliged to warn you away from turning your characters into enormous bundles of unique quirks. It can be really tempting, but the results aren't pretty. Beyond that, take all these guy's advice.

Click to expand...

This also. Don't introduce quirks for the sake of having quirks. Have the character be who they need to be for your story. Chances are, their personality will present itself. And honestly, you don't need to have every "spear carrier" be a well-drawn, detailed person. How often do we interact with people in our daily lives without really seeing or noticing much about them. Sometimes writing, "The waiter brought them their beers," is all you need to do.