I'm working on a novel of my own and, looking back at the bits of it I have so far brought into light, that almost everything in it is dialogue. Is this a bad thing? How could I fix this if it is?
It is hard to overdo good dialogue; but not all dialogue is written well.
Too often a great deal of dialogue is a one-sided speech about how the author feels; it sounds preachy and unrealistic, because other than actual speeches by politicians and other leaders (e.g. preachers in church), we encounter few "casual" speeches where one person is allowed to hold forth for several minutes. It doesn't sound natural. And no matter what speech is given, even by a character that IS a leader, it is probably preaching some message the author wants to get out.
Dialogue should be treated as a form of action. The setting should be fully visualized, the characters should be doing something as they talk. "Talking Heads" should be avoided, where the reader quickly gets a sense that the "back and forth" occurs in a vacuum with nothing else going on. Between lines we need visual clues, even minor actions of what characters are doing, the expressions they make, the thoughts they have and emotions they feel which they don't express.
Characters should ask questions to clarify something, they should disagree and refuse to concede their own point, or go off on a tangent because they were reminded of something similar. They should not be passive, if you find yourself tempted to write a character saying "Go on," or "Tell me more," your dialogue sucks. (Some exceptions, a psychiatrist might say that to a patient, or a girlfriend to another girl after some revelatory statement:
Susan said, "Okay. I went out with Josh last night."
Cheryl was dumbfounded. She waited for Susan to say something else, but she didn't. "Go on!"
Susan laughed. "Well ... Not disappointed!"
Avoid talking heads, avoid passive participants, avoid speechifying, avoid predictability, it is boring. Find tangents and disagreements, don't be afraid to write dialogue that ends in a stalemate. Dialogue should have conflict. That is what keeps the reader reading; they want to see how it turns out. The dialogue should matter to what happens next. Even if Josh is declaring his love for Susan, the reader should see Josh is taking a risk and Susan may not reciprocate; they should be waiting to see what Susan says or does, or how she feels, or what Josh is feeling in the interim. (So what happens before the dialogue matters here, too.)
The length doesn't determine whether the dialogue is good writing or bad writing. Good writing is interesting and fun to read, it has unexpected moments (but they make sense). A long dialogue can accomplish that. Bad writing is boring and predictable and doesn't seem to matter to what happens next. A long dialogue can be that instead.
The problem with too much dialogue, is that dialogue alone doesn’t create a picture.
A story should immerse the reader in a scene, and that immersion should be so complete that the reader forgets they are reading. To do that requires a balance of all the elements that make up a scene (setting, dialogue, interior monologue, non-verbal cues, physical action, etc. etc.).
I think the best way to grasp it is to see every individual scene as if it’s the loading program in The Matrix. If you have nothing but dialogue, Neo and Trinity are standing in the loading program having a conversation. There’s nothing else there except empty white space. If you bring in setting: a chair, wallpaper, carpet, etc. you’ve made a start on creating the room they’re standing in. But, conversely, if you go too far with setting, at the expense of other elements, you still just have a pretty room and a conversation.
You can dive into Neo’s head with interior monologue and get an idea of how he feels about the dialogue. You can have them move around, sit down, use non-verbal cues and so on. But only the things you’ve written into that loading program exist for the reader.
So, the reason dialogue alone doesn’t work is because, for the reader, it would be like watching Neo and Trinity having a conversation in the loading program for the entire movie. Imagine how boring that would be.
Another mistake that beginner writers often make is to tell their story instead of showing it. They have the same problem of a lack of balance, only now there’s too much exposition. The writer has created a character and sat them down in a room and made them tell their story. There’s no action. And it’s akin to watching Neo tell the whole Matrix story from a chair in the loading program. Again, it’s boring.
You have to create EVERYTHING from scratch for every scene. You have to balance every element so the reader can see the scene taking place in their heads.
It’s not easy! But it gets easier with practice.
The question isn't whether you have "too much" dialogue, but whether you have "enough everything else." There are successful novels that are composed of almost all dialogue, and very little else, but those are definitely rare exceptions. A book without rich descriptive passages is like a cake without flour. It's possible to create, but you'll working without most of what normally holds the structure together, it will feel like something crucially important is missing to most people, and some people will reject it entirely. So let's say yes, your novel is unbalanced. What are options for fixing this problem?
One option is writing a play or a screenplay (or a graphic novel script) instead. These are centered much more around dialogue and plot --it becomes someone else's job to provide the rich visual images that will help people become immersed in your fictional world. Let's assume, however, that you are committed to a novel. I have the same basic inclinations as you --left to my own devices, my writing is all dialogue --and these are things I've done that have helped me develop my descriptions:
Pay more attention to the visuals in real life. We write what we notice, and what we love. You probably pay a lot of attention to things people say, and less attention to what they look like. Practice seeing the things you usually miss.
Remember, descriptions should never be dry catalogs of visual details. Every description should come with a mood, a point of view, an attitude and an context. Is the sky paradise blue? Or pitiless blue? The actual color might be the same, but the way you describe it conveys a wealth of info about your character and helps put the reader in the character's mind.
Something I learned here, on this SE, from @MarkBaker, is that metaphors tell their own stories and carry their own narratives. "I walked through a forest of trees that stood like angry soldiers, with branches like spears." That gives a strong visual image, but it also carries its own story, one that can inform the main narrative.
It might be worth spending some time going through some description-based writing exercises to develop your descriptive "muscle." Once you learn more about what descriptions can do for you, they'll seem less like an onerous duty or hoops to jump through, and more like an opportunity to add depth and layers to your storytelling.
First Drafts, Writing and Rewriting
This is your first draft, right?
Your process could be that you produce a lot of dialog.
You can fix that when you edit the text.
To quote Hemingway (or Arnold Samuelson?): "The first draft of anything is shit."
Go back (now or once the first draft is done) and look at your scenes. Do you see things (character action, setting, etc) that are not on paper? Then add that.
Imagine what your first-time reader will see in their head (or won't see) when they read your text and edit to make that image what you like it to be.
As long as your final version of the text has a good mix of dialog, internal emotion, description, action, etc, it does not matter what the text looks like right now!
After all, writing is rewriting (also a quote from someone or several someones...)
(Case in point: click the "edited" link in the center of the footer of this answer to see my edits... or click that link on any other edited answer on Writing.SE. I'm pretty sure you'll find that the answers improve with each edit.)
So what does that good mix in a scene look like then?
You have several good answers here already (avoid talking-heads, center the scene in a room with things and people, paint a picture, etc), but I'd like to add that dialog, as well as a whole scene should have conflict.
You get conflict by having characters with opposite or incompatible goals.
If you find yourself writing a scene where all the characters have the same goals, consider putting the whole thing in a sequel instead of a scene, if it's important enough to not warrant being cut completely.
Dialog is also special in that it can (should!) have subtext.
It can, in contrary to many other parts of a text, have the characters say one thing and mean another.
Good dialog has a surface level containing what is said and done, and a sub-level where something else is going on, something that can change the meaning of the surface level or enhance it.
In order to show the dialog has a double meaning, you need more than just the words. Goals and internal emotion are most often used, but facial expressions, body tension, and action can also show something else is going on.
If one of two passionate-talking lovers is backing away there's something up with the "passion."
Not using subtext in dialog is wasting an opportunity to make the scene do double duty.
Obviously, this means that much dialog along the line...
"Let's go to a movie."
...could easily be placed in a sequel ("They went to see Endgame.") unless the subtext is along the line of...
"Let's go make out in the darkness!"
"I'm famished and would love to be near your juicy, blood-filled neck in a dark room!"
...hinting at conflict due to pretty incompatible goals...
Do you think it is possible for a reader to follow your story of 80,000 + words simply by writing the whole story in the form a of dialogue between or among characters? I believe it would drive most readers to distraction. Try this: put your story in a drawer for a few weeks. Then take it out and read it. If you hear everything you wanted to say then you are on the right track. If you get confused, then try describing what you want the reader to feel.
Welcome to SE.
Others have said this, but it bears repeating: The purpose of non-dialog portions of a story are to provide context, setting, interiority (thoughts, emotions), and so forth. Too much dialog means you probably have too little of something else.
Of course the approach can work... See the first chapter (Chapter A) of New York 2140. All dialog, almost all of it, and a grand total of only two or three attribution tags, but it works. The writing is fully intentional on the part of the author, and Chapter B is a different style altogether (and a different viewpoint.)
My answer, without seeing the excerpt, is: Probably you have a 'bad thing' in your draft, yes.