Dwarves are one of the iconic races that people most commonly play. With their gruff demeanors, their thick beards, their short stature and hardy nature, they appeal to anybody who wants to play a tough and assertive character. But how do you help your players from playing a caricature of a dwarf? How do you get them from playing a gruff human to actually playing a dwarf? What advice and help can you give as the DM to get them truly in character?
The most important thing to fix on is where dwarves are meant to hail from. Dwarves are chthonic creatures, denizens of the underground, spending their lives in great underground halls and mines. They rule vast underworld empires, fortresses in the deep, spending their long lives battling all manner of monsters that come up from the greater, darker depths. Thus when a player decides to play a dwarf, they need to remember their heritage, and how this colors everything a dwarf does and sees.
Imagine spending hundreds of years underground. With vast vaulted cavern ceilings over your head, with light coming only from what torches or fluorescent fungus might be around, dwarves are unaccustomed to such vast open expanses as the sky, as prairies and savannahs. It's helpful to imagine the reverse; how would you feel if you suddenly had to adventure in the depths of the earth, moving through tight tunnels, always ducking your head, in darkness, the weight of the earth always pressing down upon you? While the years can help ameliorate this difference to a dwarf, they will always be more comfortable inside a house, or in the tight streets of a warren like neighborhood, or even under a dense canopy of great trees. Out in the open will make them uncomfortable, irascible.
Further, dwarves are creatures of oaths and honor. They spend their lives toiling beneath the earth, mining and crafting, building and working wonders with their hands. Think on that: they eschew shortcuts, the easy way out, the quick and dirty. If it's worth doing, it's worth doing right. A thing built right will last for centuries, if not forever, and a dwarf is judged by the quality of his work. This honor and pride will influence everything they do, from the tasks they undertake as adventurers to the pride they take in associating with their fellow companions. Staunch and honest, dwarves judge themselves and others by the quality of their work, not by their silver tongued speeches. There lies a hint as to why dwarves mistrust elves, for what do elves love to do other than glory in nature by walking around and talking to trees?
Finally, think on dwarven avarice. Setting great stock in things of beauty, dwarves spend years crafting works of wonder and elegance. From the dark they produce treasures fit for kings, and these made of gold, silver and more. What kind of creature values treasure in such manner? There is something of the dragon in them, the fascination with objects of wonder, the desire to possess, to own. This is where darkness can enter their spirits, that can lead them to thievery, adventure, the kind of life that is far from that of honorable toil deep in their mountain homes.