Dsspite being, if I do say so myself, very varied and different, my characters all speak the same. That is to say, like me.
They speak the same? Or they are the same? I'm not sure how they could all speak the same without being similar, but if it's just a matter of quirks and speech you could simply add something unique to them, for instance have them have their own sayings of sorts. If you've ever watched Doctor Who, something like Allons-y or brilliant, something not really a lot of people say. You could also have speech quirks, for instance I was in a roleplay with a character that pronounced everything as if they were questions (which was by the way super annoying, but a really interesting quirk.) Or you could go with something simpler, like not pronouncing a certain letter etc.
It's hard to get out of your own way sometimes, I like to add unusual speech words or patterns as well. I find it fun to pretend I'm British or Australian for a day and think about how the would turn a phrase
I've never gotten the hang of wildly different speaking styles, but I can fake it a bit with different vocabularies. A very educated character might use the broadest reaches of my vocabulary, while a young child might only use the simplest words I know. Add in a few quirks like profanity or a tendency to cut off leading words, and readers are surprisingly willing to suspend belief. Honestly, I think a lot of great writers can't do very different voices. For instance, every character in Oliver Twist has noticeably different speech quirks, but they all recognizably fall under a general "Dickens voice."
I feel that as soon as your characters become 'real people' to you, their individual voices will appear and more or less write themselves. Think of real people you know. They don't all speak like you do, I reckon. Some will speak faster than you, some slower. Some won't make a lot of sense until you get them to clarify what they mean. Some will rarely answer a question directly, but will kind of hint at stuff instead. Some are in-your-face blunt. There is lots of variety out there. See if you can base a few characters on people you actually know, and see if that helps any. Problems can come when a writer becomes so focused on moving plot along that the characters simply become devices to make this plot happen. What gets said becomes the only thing that matters. If you focus on how and why your characters are saying what they do, then I think your problem (if there is one) will iron itself out. There will also sometimes be subtext (stuff that is implied but not said directly) that is worth building as well. Don't artificially create speech quirks, because that artificiality will show. Instead, use your imagination to discover your character's individuality, listen to them, watch them—then write how they would actually speak. It's easier than it sounds, IF you take the time to let your characters come alive in your head.
My suggestion: 1) Pick rules for how each character's dialogue, observations, an internal monologue (some characters might speak/think in shorter sentences while others drag on longer, some characters might swear more or less than others...) 2) Write a character naturally, then change the word choice so that you satisfy your rule perfectly even if the sentences don't feel like the way that a real person would speak naturally anymore. 3) Change the sentences again so that they still meet your rule almost as closely, but in a way that feels real and natural again. 4) If you decide that a rule you've come up with for a character doesn't work as well as you'd thought it would, drop the rule and try something else. 5 and most importantly) What are you already doing to distinguish between your characters in addition to just their word choice?
I give my protags various speech characteristics. For example, one individual will repeat himself. "Go out? Go out, you say?" Like that... While another has a very working class, way of speaking. "Like wot I said, last time. Get it?" So I vary their speech somewhat. This is not the case for everyone, all of the time. But when possible, I give them habits that help maintain their character.
It's funny. During my many edits of my novel, I would often rejig or even remove a line of dialogue from a character, because "he would NEVER say that!" Or he'd say that, but not so informally. Or he'd say that, but not to her. Etc. In other words, the better you get to know your characters, the more likely their dialogue will sound either right or wrong to you. Adjust accordingly.
It would be helpful to write character outlines for each person. This way you can distinguish the differences between backgrounds, traits and more. If someone came from no family they may be more quiet or reserved. Someone from a different area of the world may have an accident or a different way of saying a word.
Sounds like the character traits are not developed in your characters well enough. You should feel one or two traits strongly. That should guide the way they talk. Watch one of your favorite movies or sitcoms. Or, think about someone you know who always getting in trouble. Remember what they do? Remember what they say? Try to come up with a situation they haven't been in. Think about what they would do and say. Act it out. Now ask yourself, what character trait makes them act that way? Now you can use that trait when you want. Talk out loud. Create some dialog between two characters in a conflict. They don't have to be mad to have a conflict, they just need something badly. Try it. One smart, one dumb. One haughty, one humble. One weak, one strong. One in need, one that is selfish. Hope that helps.
A while ago I read an advice (I don't remember where so no source unfortunately) which suggested that you go out for a cup of coffee and just listen to people around you. Basically be creepy. This can be applied in my ways such as listening more carefully to how your friends talk rather than what they say... That, though, is just asking for an awkward situation when someone asks for your opinion.
I've had the same problem, noticing that my characters all have too much in common with me. I find that I can write someone's dialogue if I have a real person that I've based them on fresh in my memory. I write out all of the action, then hone in on the character's personality. I pick a character, find someone they're based off of and put myself in their shoes for a little while. Got a witty sarcastic character? Watch an episode of Friends and take on Chandler's persona. Got a soft spoken genius? Check out a lecture by Carl Sagan on Youtube. If it's fresh in my mind, I find it easier to write a character, I'm sure you can find some character in TV / youtube that you can relate your character to.
There was an exercise we once had to do in a screenwriting class that really helps with this. Go to a public place with a pen and pencil. Sit down near another group and write everything they say in their conversation. It'll give you a sense of different rhythms and mannerisms for how different people talk. It is a huge help for getting better at writing diverse dialogue.
