Not only do you need both, but it's important to know what to put in and what details to leave out in terms of both dialogue and details. To illustrate dialogue:
Nuh-Uh:
Kim leaned in, frowning with concern. "Ron, are you all right?"
"Kim," said Ron, "Let me explain, in painful detail, the nature of my feelings for you, and why this heartache is causing the adverse physiological reaction you see before you."
"Oh...okay," drawled Kim, shying back and eyeing the gathered throng in Bueno Nacho for an opening to facilitate her quick exit.
Yuh-Huh:
Kim leaned in, frowning with concern. "Ron, are you all right?"
Ron glanced up. "Huh? Oh, yeah. Sure, KP, I'm just..." He paused. "Yeah."
See? In the first segment, Ron sounds like he's been possessed by the ghost of a Victorian-era novel hopped up on expositorium (which we'll assume is some sort of opiate that makes characters long-winded). He's out of character, has the wrong voice, and worst of all, he's spoon-feeding his readers information which they should be discerning via other details, which we see in the second segment: Ron's clearly feeling uncomfortable, enough so for Kim to notice, and it's something he doesn't want to talk about with her. Could it be feelings? Ding-ding-ding!
Out of Character is one of the most rampant problems in fanfiction, and can easily turn a reader off to a story. I can't tell you how many times I've clicked that ol' back-button on my browser when Ron starts using words (like the abovementioned "physiological") that he couldn't spell, much less work into an actual conversation. Luckily, it's one of the easiest to fix as well. Writing dialogue is a simple, step-by-step process:
1. Figure out what your character has to say. What information does he need to convey? Is there any way you can convey it without dialogue?
2. Factor in how your character feels. Is he nervous? Excited? Sad? Hungry? Whatever his mood, it'll affect his delivery.
3. Know your frikkin' character. Know what kind of words he uses. You can't start writing a dramatic monologue for Ron, and then toss in a "Booyah" just to remind people that it
is Ron. What other kinds of words does Ron use in specific moods? How does he speak to certain characters? You'll notice, Ron talks to Mr. Dr. P with a certain respect that he doesn't usually afford other adults.
4. Put it all together. And remember, keep it simple and straight-forward. A good clue is to start reading the dialogue aloud. If you could never imagine it coming up in a real (or even fictional) situation, it's a good bet that you've got a stinker there. Next, try to imagine your character saying it. Imagine his voice in your head. If it sounds like him, the line's a winner.
But others noted, dialogue is only half the battle. Unfortunatley, how much description you put in depends entirely on your own style. There are, of couse, certain mistakes I always like to avoid, which I'm willing to share with you now, in my boundless, arrogant generosity:
1. Important details are the only details that should ever see the light of day. For this, Tolkein is a great example of what
not to do. If you've spent three pages describing a tree that your characters pass, that tree darn well better be the crux of your story; fight scenes, love scenes, tragedy. Hell, the tree should be the main character at that point.
Remember, if it doesn't add anything to the story, don't put it in. This means extraneous characters, descriptions, objects, etc. I don't care if you promised to put your friend in your next story; If he or she isn't needed (if there's already a character that exists to do the job), leave him/her out. You think it'd be cool if Ron had a Jaguar? Well, it'd better be important.
2. Long segments of dry description make for bored readers. Instead, try to describe your setting, or an object, by having a character interact with it.
Nuh-Uh:
A smooth metal sphere sat atop Mr. Dr. Possible's work desk. Its glossy surface warped the room's reflection into a funhouse mirror. Some kind of power radiated from the sphere, an intangible warmth despite it being cool to the touch. It weighed about as much as a bowling ball, though it was no bigger than a baseball.
"Neat," said Ron as he lifted it from its place on the desk.
Yuh-Huh:
A glint of metal caught Ron's eye and turned his head as he passed Mr. Dr. Possible's desk. "Neat," said Ron, as he picked up a small, silvery sphere. It carried with it a surprising heft, though it fit neatly in the palm of his hand as he raised it to his face, and tingled with an intangible heat just beneath his skin. A funhouse-mirrored Ron smiled back at him from its cool, rounded surface.
The details given in both sections are the same, only in the second, we discover everything we need to know about the sphere right alongside Ron, giving the reader the illusion of taking an active role in the story. We'll discover even more once Ron drops the device and finds out what it does. ;D
3. This happens to be a personal favorite of mine, and the core of my style, so don't steal it too much
Rather than describe the character in a setting, try describing a setting affecting a character.
S'okay:
Kim felt a stiff breeze start up as she walked home.
Me-Likey:
A stiff breeze ruffled Kim's hair as she walked home.
Little things like that can breathe new life into an otherwise boring segment.
I hope some of this helps. Remember, these are just tips. In the end, you'll have the final call to make all on your own. Lord knows I break these rules every now and then, whether I mean to or not. But they sure can't hurt. Good hunting!
Cyberwraith9
Ghost of the Net
and your Queen of Fanfiction