One important thing to keep in mind about writing a novel is balance. Not balancing your writing with the rest of your life (though that is important), but finding balance within a story. The key elements of fiction writing are action, dialogue, description, thought/internal monologue, and emotion (not thinking about feelings, but actual physical feelings, like racing heartbeat, lump in the throat, etc.). Too much of one of these things tends to bog down a book.
In a novel, you want a lot of dialogue. Dialogue creates white space on a page and leads your eye down the page to keep reading. Pages and pages without dialogue can look daunting and dense. Dialogue generally reads faster than blocks of text. Also, if there's dialogue, that means there's more than one person in a scene, and they're at least doing something -- even if that's just talking -- rather than sitting and thinking.
But while dialogue is your friend in a novel, you need to have action, as well. Talking heads -- people doing nothing but sitting and talking -- are pretty boring on television and in movies, and you seldom see a scene that is nothing more than just a conversation unless that scene is so emotionally charged that it becomes compelling on its own or unless the scene is laden with subtext, so that what they're saying is in contrast to the non-verbal message. The same thing applies to books. We want scenes about people doing stuff, not talking about stuff. They can talk while they do other things, but a scene that's just talking needs to have something else going on to make the talk interesting.
Thought/introspection/internal monologue can be tricky in a novel. It's the key difference between visual media and books. This is our chance to get into a character's head and understand a person from the inside out. There's no introspection in a movie unless there's a voiceover or the character talks to herself out loud because we can't get into a person's head. All that stuff has to be conveyed in other ways, such as through action, dialogue or acting (facial expressions, non-verbal communication). One reason I used to love movie novelizations was the chance to get into the heads of the characters and read their thoughts. But too much introspection can bog a book down. Do we really want to spend three pages with a character mulling over a decision? It can be a good exercise to think like a screenwriter and see if there's any other way at all to convey that information before resorting to introspection.
Description and emotion are the seasoning of a story. You don't really want huge chunks of it, just enough to give it flavor at the appropriate point. It's best to mix these things into other elements.
The precise balance of these elements will depend on the kind of story you're writing. A romance novel without an external plot is going to have a lot more dialogue and introspection than an action novel would. Romance readers are looking for character interaction and thoughts about feelings, so you'll want to shift that way in that kind of book -- but you still probably don't want that many talking heads. Fantasy readers expect a fair amount of description if the story takes place in another world, so those novels might have bigger amounts of description, especially near the beginning as the world is established.
One way to get a sense of the right balance is to get an extra copy of a book you enjoy in the genre you're trying to write and maybe a recently published and successful book in that genre (but not one by an established bestselling author because the rules don't really apply to them) and a set of colored highlighters or pens. Create a color code for the various elements and go through highlighting or underlining the elements as you find them. If you don't want to do the whole book, do key parts like the opening, midpoint, and ending. Then you can flip through and see at a glance the balance of dialogue, action, introspection, description, and emotion. Next, print out your manuscript or portions of your manuscript and apply the same color code. How does it compare?
I've also made lists of scenes in a book and decided if they were "talking" scenes or "doing" scenes, color coding them, and then deciding if the talking could be folded into doing or if the talking was important and compelling enough on its own. That was because the amount of dialogue looked about right until I realized that all the characters were doing was talking, and that's not good. I've had editors say they'd rather not see any restaurant scenes, even in a romance novel, because that generally means there are just two people sitting and talking. Of course, if the Mafia bursts in and takes hostages or if the place burns down, then you can have a restaurant scene.