This is a portion of an article I found on Roleplaying Tips for the entire article go here. I thought it was really interesting, from  GMing perspective. I mean how often have you described a monster to your group to only have them go, " Oh, it's a so n so ", and then proceed to do exactly what they have to do to take it out. Even though their characters had never encountered one before.

How To Get More From Your Monsters - Part 1

By Andrew H.

Many of the following tips have been suggested before, over the years. I hope to give you a practical spin that will enable any game master to turn a single adventure into a whole campaign. I have used these techniques in my campaigns and found them to be effective.

1. It's Never An Orc

It's a Greater Georlocke to the locals, a Drop Bear to foreigners, a Kellerman's Fiend to the Duke, a Tribal Gengen to the sage, and a Thorgordrin to the nearest clan of elves. If any two players can agree on what to call it, they still won't know what it is. The aim of this approach is to keep the monster's game stats unknown to the players. This helps keep the mystery and sense of discovery in the game.

The work involved to make this worthwhile is to craft a set of descriptions, rumors, tales, and folklore to flesh out the monsters. This can be done with an extra ten minutes of design time.

The players should know only what NPCs can tell them about the monsters. It shouldn't be an orc, it should be:

"The Dark Terror of Ovens Lane that took Fred the Smith last year and was stalking Hilda last month."

"When Billy the Farm Hand was injured last summer he took longer than he should have to recover since his wounds got all infected."

Just one rumor about poison or magic will make the players approach the fiend differently than they would an orc. This is a great opportunity for both the GM and the players to roleplay.

Players will not believe there be vampires here if they are first level, as they could not be expected to fight such a powerful monster. But if they hear something more in line with their own strength, they are much more likely to believe, and act accordingly.

How This Worked For Me

Like many GMs, I invent names that sound like they belong to a particular language. Elven names are fluid, musical with softer consonants, etc. I deliberately chose a name that sounded like an elven name and used it as the local name for gnolls. Then I added some eye witness accounts of a raid on a merchant caravan. The players were convinced they were facing a new monster I had designed, and they spent time thinking about the enemy's tactics, strengths, and weaknesses. The players approached the monsters more cautiously than they would have ordinary gnolls! Players will appreciate this approach, especially if they get experience bonuses for good roleplaying based on what their characters have heard about the fiend.

Give bonuses for good ideas and preparation. If the PCs thought to bring some antidotes with them, reward them with a small experience bonus, even if the orcs don't use poison- -they are playing their role well and thinking (always encourage thinking)!

Adventure Hooks

• A sage suggests to the players that, if they could find the true name of the monster, they would find out its weaknesses and be able to defeat it. Turns out they only need to locate the lost manuscript of the sage Alvin Cork to find out.

• Two party members cannot agree on what the real name and nature of the monster is. They decide they must travel to the homeland/town of each of PC and consult their elders, sage, grandmother, or whatever, to come to the truth of the matter.

• A sage hires the party to help her write a treatise on the monster in question. Either the party has to make notes about the monsters, characteristics, combat strategy, and abilities, or worse, they have to babysit the sage as she comes along to take her own notes. Of course, they don't get paid if she doesn't come back alive.

• The deceitful Duke refuses to pay up his promised reward for ridding his lands of the fiend because it is not a Kellerman's Fiend but a Gengen. Ask any of the villagers.

This is an article from Helium written by Matt Bird. It is more applicable to D&D 3.0/3.5 and Pathfinder RPG than the current version of D&D.

Your typical Dungeons and Dragons PC will run the gamut from lawful good to chaotic neutral. Most bands are composed of a mixture of good to legally-ambiguous guys and gals who want to put an end to evil in the world. These groups are probably the easiest to handle and participate in, for though the methods may be different the ends are usually the same.

But that narrow view of Dungeons and Dragons eliminates three alignments: neutral evil, lawful evil and chaotic evil. These simple choices can dramatically change how a character acts.

At its basic an evil character is easy enough to understand. They're the bad guys. They work to make everyone else miserable, thinking only of their own goals and ambitions. And, typically, they don't wind up in a group of good guys. Evil characters are on the other end of the sword. Or they would be in a black and white game. Dungeons and Dragons is seldom black and white. There are plenty of morally-ambiguous situations in which good and evil can come together to complete a common goal. And though they don't always get along together - indeed the union may result in eventual disaster - it's perfectly reasonable to expect a group with good and evil characters to eventually succeed.

That is, so long as the PCs don't act TOO evil.

Playing an evil character who's true to their nature will require some evil acts. Villains have little by way of scruples and will cheat, lie and kill to get what they want. This usually contrasts with the beliefs of good characters who, understandably, loathe harming the innocent (and even those who aren't so innocent, or at least not for a good reason).

But that doesn't mean you need to be overtly evil all the time. Think about villains in real life (or people you perceive to be villains, anyway). Do they constantly wander around killing people in the streets? Is their every last act tempered by malice? Or can they hide their temperament when they realize it's a dumb idea to be evil? Likely the latter, if they want to remain free.

And you should act similarly if you want to play an evil character. By all means, kill people if you're evil. Steal. Cheat. Lie. Slander. But don't parade around committing evil acts all the time. Don't get your party in trouble just because you're 'playing in character'. In short, don't be an idiot about being evil. Be sly, be opportunistic and, above all, be discrete. Carrying out vile acts behind the backs of your allies will ultimately be more fun than turning into a ravenous axe murderer, especially if you want to survive more than one session.

One last practical tip: if you want to go evil, you may want to avoid chaotic evil. Chaotic evil characters tend to act on their impulses on a whim, doing as they please when they please. There's a reason most monsters are chaotic evil. Go with neutral or lawful evil and you can create a much more sociable, balanced character who can interact with goody-goodies while remaining true to your vile nature.