The following is text from the Shadowrun rulebook (copyright 1989), from FASA Corporation, written by Bob Charrette, Paul Hume, and Tom Dowd. This is selected text which reveals the authors' view of what roleplaying is and the responsibilities of GM and players.
"BEHIND THE SCENES" Chapter introduction, page 152
Behind the scenes is where you find the person who makes the game happen. In other words, the gamemaster. The gamemaster has many functions, including creating adventures, roleplaying NPCs, and mediating rules and other formalities of play.
To create adventures, the gamemaster can make up the story out of whole cloth, use a Shadowrun adventure published by FASA, or adapt a plot from fiction, TV, or the movies. In practice, many gamemasters end up mixing all these elements to create an adventure best suited to their own and their players' interests and personalities.
The gamemaster plays the roles of all the people, computers, critters, or anything else the player characters meet. The Contacts section profiles a nmber of 'typical' NPCs from 2050 to get you started. FASA's Shadowrun adventures will give you more. Sometimes, it's fun to use a published adventure as a starting point, creating your own NPCs to replace the published ones.
A good gamemaster is always open to discussion about how the rules work, but when he makes his decision, it's final. As gamemaster, only you decide how much Karma to award at the end of each adventure.
That sounds like the gamemaster has absolute power, but it really depends on how much power your players are willing to give you. If the players don't like the way you run the game, they won't play it with you. On the other hand, if you don't like the way they play the game, you won't run it for them. The moral of the story is that conducting a roleplaying game requires some cooperation from everybody!
"BEHIND THE SCENES" Chapter, "TAILOR THE ADVENTURE" section, page 158
When getting a group of players together, it is best to learn something about their interests or else your adventures could fall flat. It's important that the mission you have planned is one they'd really like to tackle, but don't expect a lot at first. The players probably know less about the ways of the game universe than you do and may have only vague ideas about wanting to make money, take on a corrupt corp, deal with elves, and so on. Once they get a few runs under their belts and their characters' life stories take some shape, the players' goals and ideas will become more defined. They may want to hunt down a particular enemy, find a lost love, take revenge on a corporation that did them dirty, or find a specific teacher or piece of custom gear. You can and should build these ideas and suggestions into major themes in your adventures.
Encourage the players to write out their character's histories, including background on family, friends, and previous employment. You can then draw on those histories to develop adventures that get everyone involved.
Be Aware!
You have to keep a lot in mind. Listen to what the players say. Keep track of your NPCs' whereabouts, plans, and so on. It's good to have a note pad handy for jotting down memos to yourself as the adventure rolls along.
Be Knowledgeable!
A gamemaster must be familiar with the whole game. That doesn't mean memorizing the complete rule book, but you should be familiar enough with it to find a particular rule or rules quickly. You should also have a good knowledge of the basic game systems.
Keep a written outline of your adventure where you can refer to it. As you gain experience, you'll probably improvise more and more detail, but for starters, it's best if adventures are simple and worked out beforehand.
Be Fair!
Stick to the rules. If you or your players hate something we've written, change it. If you do that, make sure everyone knows what the new rule is.
Remember that you, as gamemaster, know much more about what is happening than the NPCs do. You may know the player characters' skills, weapons, spells, and so on, but the NPCs don't. It would be unfair to let the NPCs behave and make plans based on that knowledge.
Be Realistic!
Remember that the NPCs that you play are just people, with all the usual fears, needs, hopes, and desires. By giving them life, the stories that come out of the game will be more memorable for everybody involved.
Play critters like real critters, too. For example, most animals have little or no interest in killing for pleasure. They fight when they need to -- to eat, to protect their young, to save their own hides.
Be Flexible!
If a player want to do something not explicitly covered in the rules, don't just refuse on principle. There is always a skill rating of some kind that you can use for a Success Test. Tell the player what skill or attribute you think applies to the situation and whether his chances are good, indifferent, or terrible. You don't have to tell him precise Target Numbers, just whether or not his goal is possible.
If you come up with a rule to cover a special case during a game, decide later whether it will become a new "house rule" that always applies in future games or a one-shot that may or may not be used again. The middle of a shadowrun is no place to discuss the fine points of game mechanics.
Be Tough!
Challenge the players. If they don't sweat to get that Karma, they haven't earned it. Two goons armed with baseball bats would not be the only security guarding the corporate data center, and the local Yakuza won't keep their main database on a home computer.
Once you and your players are comfortable with the way the rules work, you'll be able to fine-tuen the "threat level" of an adventure. For now, keep in mind that on a really rough run, the player characters should, ideally, win it by the skin of their teeth, if they win at all. If you don't get it right at first, nobody's gonna fry you.
Be Kind!
How can you be tough and kind? As gamemaster, you can kill off a character anytime. You can throw enormous risks at the player characters until their luck runs out and they fail a Success Test. But only cheap bullies do that. Gamemasters who measure their success in trashed character sheets soon find themselves without players. Better to be too easy on the characters, rather than too deadly.
When the player characters get in over their heads, remember that bad guys like to take prisoners. Prisoners can be make to talk. Prisoners can be used as hostages. Prisoners can also escape or pay ransoms. Most important, prisoners have a chance to escape and live to fight another day. If fictional villians were smart enough to kill off the heroes at the first opportunity, then all the adventure movies ever made would end after the first ten minutes.
Attributed to the 2nd edition (page unknown):
Everyone has read a book or seen a movie where the lead character does something that the reader or viewer finds so utterly wrong that he or she wants to yell out and warn them. But whether the reader calls out or not, it makes no difference. No matter what we say, the character will do what the plot demands; we're just along for the ride.
The situation in a roleplaying game is very different. When roleplaying, the players control their characters' actions and respond to the events of the plot. If the player does not want the character to go through the door, the character will not. If the player thinks the character can talk him or herself out of a tight situation rather than resorting to that trusty pistol, he can talk away. The script, or plot, of a roleplaying game is flexible, always changing based on the decisions the players make as characters.
The person controlling the story is called the gamemaster. His or her job is to keep track of what is supposed to happen when, describe events as they occur so that the players (as characters) can react to them, keep track of other characters in the game (referred to as non-player characters), and resolve attempts to take action using the game system. The gamemaster describes the world as the characters see it, functioning as their eyes, ears, and other senses.