The big beginner's mistake is to ladle on description by the potload:
The tax inspector, Grindle, was a small, dark-skinned man with a huge nose on which perched a tiny pair of rimless glasses. His wavy grey hair failed to conceal enormous ears that flapped on either side of his head like a warning system for a nuclear winter.
He wore a tweed suit, the kind of suit you'd imagine worn by the first Ministers in the Free State parliament, devoted to buying Irish produce and heroically bearing the heinous scratching of their inner thighs by the hardy produce of mountainy looms.
His shoes had the mellow shine produced by daily saddle-soaping of the undyed pigskin; somehow Ferber imagined that he could see remnants of the soap caught in the patterned holes on the brogues' toes.
He had the scent of a subtle, expensive but not too expensive aftershave balm.
Even his briefcase glowed.
When he spoke, it was with a preliminary throat-clearing. "Mr Ferber?" he said, in a voice that rubbed its hands with every syllable.
About halfway through the second line of this stuff - all useful stuff, by the way - the reader is already mentally moistening a thumb, ready to turn the page, while the unconscious mind is jumping up and down pulling at the reader's sleeve and screaming, "WHERE'S THE STORY????"
And this is the important thing for every writer to remember. That's the question that every line must answer: where's the story, and what's going to happen next.
This kind of massive infodump doesn't move the story along at all.
Another basic mistake in the wodge above: whatever you do, be cautious of adjectives of size. Every time you use big, small, large, wide, minuscule, loud, soft, and all their evil friends, remember that the concept you reach for first may not be the strongest one.
If you want to get across the fact that Mr Grindle is small, for instance, the most powerful way to do it is through action:
Ferber looked down at Grindle. Grindle stepped back and looked him up and down, as if to say that anyone over six foot was a lumbering boor.
This brings us to subtext; every statement ever made has at least two meanings: the overt one, and the one that carries the action. Here, you've got Ferber trying not to irritate Grindle by towering over him, and Grindle making a power play. And you've described the size of both, and given an impression of neat little Grindle and big awkward Ferber.
Dialogue is a shocking temptation for fiction writers. It's a nice easy way to carry the action, and you can go on for pages at a time. If you don't want readers, that is.
The thing to remember about dialogue is that less is more. A good - short - dialogue is like a close-up, where the reader's vision moves right in to see the characters with every open pore in clear view.
Grindle pulled a gun. "Gimme the cheque," he rasped.
"The cheque?" said Ferber.
"Yeah, the cheque. I've got a private jet booked for the Bahamas," he snarled. "And I'm running late."
"But... but..." protested Ferber.
It's important in the commentary ("he said", "she whispered") to avoid allowing these phrases to take over the dialogue. If you find they are becoming intrusive, it's usually a sign that your line of dialogue is too long.
Young writers have been taught a passionate hatred of adverbs. There used to be a happy game called Tom Swift, which consisted of fitting horridly apposite adverbs to a statement:
"Gotta run," said Tom swiftly.
"I could eat a cow," said Tom hungrily.
"Is that a violin?" asked Tom musically.
...and so on, mocking an early 20th-century usage. Nowadays adverbs are regarded with such delicate horror that apparently some (probably second-class) agents simply do a global search of any submitted manuscript for words ending in 'ly', and if they come into thousands, the agent takes the manuscript in finger and thumb, drops it delicately, carefully, distastefully into an envelope and immediately writes a rejection letter redolent of horrified refusal like a country virgin of 40 who's met the Devil at the crossroads outside the graveyard.
(Which reminds me of such a story in which the Devil made a shocking suggestion involving oral pleasures to such a virgin, who was on her way to Midnight Mass; "Oh no, I couldn't," she said, "I'm receiving.")