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Roleplaying well is a very difficult thing to do. Many players are embarrassed about actually speaking in game as their character would speak, and thus laugh or turn their characters' speeches into jokes. More players simply do not know how to roleplay, and after making half-hearted attempts to do so, simply fall silent and wait for the next battle. The good news is that none of these behaviors are the fault of the players. Perhaps no one has ever told them how to do it; perhaps they aren't aware of how much the fun of the game could be increased by good roleplaying. Let's see if we can fix these two oversights.
There are two major things to keep in mind when endeavoring to roleplay well. The first is to take a lesson from literature and the movies and have a character arc. A character arc is the journey the character undertakes over the course of the campaign. No, we aren't referring to the journey from the tavern to the dungeon—it's the internal journey, the lessons of morality and knowledge that the character learns. The character could start as a generally good person and become corrupt when faced with temptations; she could start out as an immoral and greedy scoundrel but see the light goodness brings to a person's life. She could start as a sniveling coward and grow into a stalwart warrior, or develop phobias of certain monsters or situations based on her experience. The possibilities are vast, and they need not be overly dramatic or involve an alignment change, but a change of some sort ought to take place. With some notable exceptions, a static character is a boring character. On a related note, players need to let characters evolve naturally with the story. Unlike in literature, the player is not the only person fashioning the story; he needs to take into account another, very powerful influence on the tale in which his PC is one of the main characters—the DM. Although the player should have an idea where his character is starting and where he wants his character to go, if the DM throws a wrench into the works—as he surely will—the player needs to be prepared to take it into his character development and possibly change the evolution of his character in reaction to it.
The second major thing that will affect a player's roleplaying is knowledge of his character's motivations. What makes the character get up in the morning? The reasons need invariably to be complex. Does he value wealth? If so, why does he value wealth? Perhaps he has a deep-seated fear of being destitute and helpless. This is the extra step that most players do not take in constructing their characters. Outward, broad motivations themselves have motivations, often grounded in the early experiences of the character. It helps here to have developed a detailed backstory for your character from which some of his incentives may spring. If the player knows these deeper motivations, he will know how to have his character react to situations as they arise. He also needs to remember that as the character evolves, motivations may very well change. If he has developed his character to a certain level of power, he may discard the motivation of acquiring wealth and come to believe that helping others, thereby amassing a large base of friends and admirers, is the key to security. Of course each character (like each real person) has various motivations, each of different strengths, and the player should attempt to develop several motivating factors for his character, perhaps numbering them according to how compelling they are to the PC. It is true that developing motivations beyond the hackneyed old standards is an extremely difficult thing to do, and the player should feel free to let some motivations develop naturally over the course of game play.
These are the overarching things to keep in mind to steer the player on the path to good roleplaying, but it is often the day-to-day, small things that are most memorable, and which will often leave the largest impression on the DM. There are four specific tips in this category. The first tip is, again, to take a page from literature and imagine your PC as a character in a novel. How do characters in novels and movies react to situations? Think about this, and then have your character react similarly. The beauty of this tip is that you do not need any other PC or NPC to react to you to engage in roleplaying: you are essentially roleplaying with yourself. For example, if your character is attacking an orc, scream out, "Die, foul thing!" If struck in combat, gasp in agony or intone, "the pain!" Give your sword a name, then when drawing it in combat, state, "Engmar will put paid to your evil!" Or possibly develop a battle cry: "Sun Arise!" Alternately, if your DM has described a scene, use common sense to imagine something that probably exists in that situation and comment on it. For example, if your Game Master tells you that you find yourself inside a luxurious manor house, comment on the exquisite artistry of the carved mantle piece. Although your DM didn't mention such a thing specifically, he will certainly appreciate your help in developing the tableau. Nor is it necessary to monologue or spend a great deal of time on these exclamations or comments; used in the right situations, they should add to your roleplaying.
The next tip is simply to know the world in which your character is adventuring. Know some of its history, find out if there are any proverbs or sayings common to it, learn some legends, and then employ this knowledge in game. You could spout off a proverb when the situation warrants it; you could tell the other characters a local legend one night around the campfire; you could wonder aloud if certain runes you discovered could relate to the ancient kingdom of Lorindon. You never know—perhaps your DM will pick up on these clues and throw in an encounter or situation relating to the things you've been mentioning. Even if he doesn't, though, you have already increased the other players' and your immersion in the game world, and thus increased the fun of the game.
Next, avoid metagaming. Remember that although you think of your character as a collection of numbers, your character does not think of herself that way. You may know exactly what your character is capable of, but your character does not. She probably doesn't know the hit dice of the skeletons confronting your party; all that she is cognizant of is the horrid claws and gaping maws of the walking dead arrayed before her. So show some fear. You might know that the party cleric (your character's friend) has stabilized at -3 hit points and will probably survive the encounter, but your character does not without a heal check; so show a bit of shock or grief. About to enter a dark tunnel for the first time? Your character is actually entering a very gloomy and frightening place, so show some trepidation. Has your character died and been resurrected? To you this might simply mean that you have to accumulate some extra experience points to regain the level you lost, but to your character this might very well be a life-changing experience; react accordingly. Remember, you view the game from the perspective of numbers and dice; your character actually lives it, and he or she has real feelings.
If you are still having trouble roleplaying, there is one final, sure-fire tip to increase your success: pick a distinctive character trait and make sure your character displays it on a regular basis. Perhaps your character stutters when nervous. Maybe your PC has trouble thinking of witty comebacks, or even specific words in general. Maybe your PC is a profuse sweater. Perhaps your character lacks a sense of humor, so make a point to react to anything funny with a very stern face (this could itself be quite funny). You don't even need to pick something as distinctive as the above traits. The point here is that your character should have something that sets him apart from every other fighter or wizard out there, something that the other players remember. A few years down the road, when you have moved on to other characters but are reminiscing about your old PC, you want your friends to say, "Roland? Oh yeah, that was the character who constantly cracked his knuckles, right?" and not, "Roland? Oh yeah, that was the 12th level paladin."
Employing these tips should go a long way towards increasing your roleplaying abilities, and they should help increase everyone's fun. Your DM is very likely to appreciate them; he will probably look favorably upon you, since you are putting effort into the very game in which he has himself invested so much energy, much of it for your benefit. But if none of these things come to pass, these tips are at least likely to score you a few of those coveted roleplaying experience points.