Here are some examples of the people you might meet while gaming.
1) Eager and clueless. They are fascinated by the game, but they just don't understand what it's all about so they just tag along, roll the dice when they are told and tell you they had a fun time afterwards.
2) The stupid. Similar to #1 they do understand the rules, but have no idea what they are playing and always come up with weird questions and ideas. "Is there a self-service restaurant in the tavern ?", "If the aliens can't be killed with guns, maybe we need magical swords.", "Lightbulbs might scare the orcs away, or maybe there is a colour that confuses them, like a chameleon on tartan."
3) On purpose stupid. They are the people who love to tell the same old joke over and over and like to sabotage the game at every turn. "That wizard insulted me, I'm going to kill him before he gives us the spell to defeat the evil overlord.", "As soon as we sneak near the Dark Elves, I yawn loudly and ask when are we going back to the surface."
4) Evil is fun ! "I round up the survivors of the Gnoll raid and tie them up to sell them as slaves.", "That farmer dared to tell me not to look at his daughter, when we leave I set fire to his hut and rape his daughter
and some sheep.", "The aliens are helpless pacifists ? Where is my grenade launcher ?"
5) The paranoid pragmatist. Everything and everyone in the setting is part of a conspiracy to kill the player, trust nothing and nobody but yourself. "That druid offered to lead us out of the enchanted forest, he's probably meaning to take us to some horrible place where he'll sacrifice us to his evil gods, I just kill him and start hacking my way out." "That Starburst lady who just rescued us is probably a double agent for Dr Devastation trying to lure us into an ambush, I blast her."
6) The comedian. Everything is a joke, nothing is serious, play Call of Cthulhu as if it was Toon. "I make loud farting noises as the dragon begins to talk to us."
7) Captain Paradigm. Your experience comes from a completely unrelated system or medium, but you like it so much it should apply to everything. "Can I adapt one of the alien races to be a panda ninja ? Like World of Woolcraft ?", "I always play a gunner, can rangers use miniguns ?" "Can you play a wizard like the Dark Knight with the bat-costume and throwing blades ?"
8) The onanic thespian. Likes to wallow in playing a character that is at a complete angle to what the setting demands and loves to obnoxiously roleplay his character just to irritate everybody. "As soon as the orcs attack I just run away, accountants have no business being in a dungeon." "*laughs* I suggest young Dwarf, as a mere commoner that you do not address me, sir Garibaldi von Klinkanfooze III, a paladin of the shining glorious order of the moon, unless you show proper respect and address me as my lord commander. The fact that we hath quested together does not imply that we are in any way kindred outside our stations."
9) The eager beaver. Scrupulously overzealous, his default setting is "overdrive" "Five sleeping ogres ? I draw their attention !" "While Bob tries to carefully deactivate the pressure plate, I jump on it with both feet !", "I'm bored, I just run across the rope bridge and see what happens."
10) The Big Friendly psychopath. In real life a pleasant, nice person, in game he gives pointers to Satan. "Doctor Lecter, we finally meet again." And shows him a spoon, Lecter starts to scream in terror and tries to dig through solid concrete cell walls until his fingers bleed. "Just before I'm led into the execution chamber, I use my secret cybermodem to detonate the Sarin bombs I planted a few months ago, I kill the guards with my bio-toxin implant and my retainer cyberrunners lock down the place so that I can teach an important lesson to the law enforcement system of this city.", "I [censored] the little girl with the [censored] in her [censored] and the [censored] so that the liquid inside the [censored] and her [censored] is [censored] over and over through the [censored] violently while the large [censored] is forcefully rammed into both [censored] until they [censored] or turn blue, and after that I reach for the Wand of Excruciating Pain and the Mace of Parasitic Infestation." The GM is in shock, some players are throwing up, others are starting to pack and leave. "Oh, Lighten up guys, I'm only playing my character, I warned you I was lawful evil. Come on, it's just a game."
Bonus : The Einzelgänger. He only comes with a very narrow range of options and sticks to it no matter what game. "What do you mean I can only play an elf wolfrider, what about a steampunk gunslinger ?", "OK, I'm the science officer on the USS Excalibur, but I'm secretly my level 62 archmage from Dungeonworld who accidentally ended up in the Federation.", "I always play a biker type, don't tell me there are no Harleys in ancient Rome ! If not I'll be the guy who invents them."