I was critiquing a friend's novel and I noticed that all the characters seem to have the voice of my friend. Speech fillers, vocabulary, sentence structure all seems the same throughout.
How can he avoid this?
The other answers are good but they strike me as abstract. Maybe I'm a philistine, but I like my advice concrete and practical.
Different people naturally use different:
When people talk about voice, that's what they're talking about. If you want distinct voices, pull apart your dialogue and reconstruct it with one eye on the above.
Hope that helps.
(Semi-flippant PS: this method of making your writing better might feel less like magic and more like work. I would argue that this is not a coincidence.)
There are two places a novelist may find a character: inside of themselves and out in the world. The desire to write may come from many places: sometimes from a desire to "express oneself", sometimes from a desire to share what you have seen and heard in the world around you. I doubt there is any way for an inward looking writer to sound like anyone other than themselves. I think an outward looking writer will naturally pick up and reflect the voices of those around them.
I not sure if you can become a sufficiently outward-looking writer simply to fix a problem like this. Writing is driven by what interests us most, and if we find that within, that that is where it will come from. I think the outward looking writer has a far better chance of producing something that is convincing and of interest to others. You don't have to be interesting yourself to discover what is interesting in the world. But the inward-looking writer may succeed if they find something sufficiently interesting within.
What you certainly can do is to try to stand back from your characters and think about who each one of them is and what each one of them wants in a scene. Every character should have their own agenda, and should be acting to advance it in every scene. What makes characters real and distinct is far less how they talk than what they say and why they say it. If your characters are distinct people with distinct agendas, they will have distinct voices, even in they all share the author's tone and vocabulary.
He's probably already done the best thing that he can do, and that is to get someone to proof read his work.
It is very difficult to extract ourselves from our characters if they are entirely invented in our own minds, so it will be necessary to get a second opinion to ensure that the character reads as someone real and unique, instead of "Me as [Insert dominant personality trait here]".
I did the same thing when I first started my novel. My three main characters became "Me as jaded and sarcastic", "Me as shy and withdrawn" and "Me as confident and outspoken", which sounds similar to how your friend's novel reads.
If you think about real people, they all have vastly different upbringings, morals, motivations and experiences. This is how we are all so different, and sometimes it is difficult to think about how they have all culminated to make that person, so it is much easier to start with what you know (yourself) and go from there.
Whilst it is useful to see yourself in the main characters, as it helps to make them relatable and use your own feelings and life experiences to make them more realistic, it also causes the problem that your friend has, which is that the characters are all developed from the same start point, thus to an outside reader they all appear to be the same character but modified slightly.
Using different methods of character creation will generally help to alleviate this problem. I found a few things that helped to create characters that seemed more real and 3 dimensional. There will definitely be more techniques, but these were the ones I used:
The suggestion of @mbakeranalecta to base the character on someone real is a very good one. Rather than writing a character that is yourself with personality traits swapped, choose someone else who is similar to who you want the character to be, and adapt from there. If you don't personally know someone who is like that, then try choosing an actor who could play that character in a movie. Basing the character on another fictional character is also an option, but it may be difficult to see them any other way than who they are based on rather than being a unique character in and of themselves, and they may and up basically becoming a caricature of the original.
Drastically changing something major about a character can help to disassociate the writer from simply writing an extension of themselves. I realized one of my characters was pretty bland, until I decided to change them from male to female. The character's gender wasn't really important to the story, and once I thought about the character in a different light it was much easier to layer on new fresh aspects of their personality and make them much more interesting, rather than adapting aspects of my own personality.
Doing some research into personality types and why people are the way they are can be very useful to adapt characters into more rounded people. This helps tie into their background, and allows the writer to explore what things in their life may have caused the character to be the person they are. Once these things are looked into, more thought into what may have motivated the character to do what they do will help to think of them as their own character instead of an extension of oneself.
A lot of writers suggest doing exercises such as "How would this character react to being stranded on a desert island?" or dropping characters into other hypothetical scenarios to get a feel for what they would do in that situation. Personally I struggle with this technique and I feel like it doesn't work for me, so I can't really expand on how to use it effectively, but I thought I would add it as an option to look into.
Reading current fiction, I rarely can tell the characters apart by their voice alone. That is, if you take a page of dailog and remove all references to who speaks, usually all the characters speak pretty much the same.
If it is part of your writing style and artistic vision to give each character an individual voice, then asking a question here is the wrong way to begin. You will need to listen to how people speak and try to emulate that in writing. There are no ready-made recipes that you can blindly follow, you will have to develop your ear and your writing ability by practicing.
If, on the other hand, you just want to tell a gripping story, different voices for different characters are not at all necessary, and most writers don't do it.
Two things in addition to what I've seen:
The more characters you have, the harder it is to differentiate them, so you wind up with archetypes. Usually this isn't a problem, but one might have fewer characters in order to develop them more thoroughly.
Keep track of your characters typical responses. "Do we go into the tunnel?" Well one of your characters might commonly say "Perhaps," while another would say, "Maybe, maybe not," and the protagonist would say, "Are you out of your mind?" It might seem subliminal at first, but over the course of a novel it gives each character at least a tiny voice of their own.
There are famous, well-respected writers whose characters all sound alike, so if the rest of the writing is strong enough, it might not even be a problem.
But to fix it, the only real solution is this: You can only do well at writing what you love, so if you want to improve your dialog, you have to learn to love the different ways real people speak. If it's not something you pay attention to on a daily basis, it will never be reflected in your writing.