Ok, so, Dwarves. A favorite of many, many players. What's not to like? They are stout defenders of their race, heavily akin to the stone they live in. Sturdy, strong, gruff, and drunk half the time, dwarves are all around fun for player characters. However, as NPCs, they often end up being passed off as drunkards and half-lawful humans. They seem to rarely have their own feeling in diplomatic and friendly relations. I think different races of people are different races because they are different.
Dwarves are generally portrayed as Gimli from The Lord of the Rings. A mountaineer sort of people, short of stature, but broad and powerful nonetheless. A society based heavily on law and tradition, which is slow to change but quick to anger in times of need. They are normally stalwart allies of humanity, and despite a slight antipathy, tolerant of elves. Dwarves are also commonly portrayed as holding an extreme hatred to the savage races, since they share their mountains with orcs and goblins. Giants in particular are hated. Dwarves seem to favor the drink a bit, which suits a people with such hardy constitutions. They are also usually shown with a flaw of greed. Dwarves value alliances and friendship, but things like that tend to crumble when glittering gold and shining gems are involved. See Thorin and the Arkenstone. See the reclaimants for Moria, who basically marched into certain death for the possibility of leftover mithril.
I think that these qualities should show through in more than just roleplaying an NPC as gruff, or mentioning that he has a beer tankard in hand. I think that one should consider dwarven culture when dealing with dwarves. Let's place an example scenario here. The PCs are all non-dwarven. Let's say that they are humans for simplicity's sake. They are headed to a dwarven city in order to establish trade relations. Perhaps this is a fairly newly founded city, a few deep tunnels dug, and some basic amenities. This means that the dwarves are more than hopeful that their human neighbors will find them worthy of a proper trading partner status. There are many goods that humans make with more proficiency than dwarves, and if nothing else, the humans will provide decent allies against the inevitable goblinoid raids.
Of course the humans would be treated with preference. Greeted warmly, not gruffly, and escorted through the nicest parts of the halls. The dwarf assigned to helping them around would likely be the most congenial, and more importantly, most tolerant of the dwarves' number. Because the humans are probably like modern day Americans in foreign lands. The average human, and probably even the party, doesn't bother to learn the culture of the place they are visiting. They don't bother to research the rules that society as a whole there just understands intrinsically.
For instance, our party here is invited to the Steelfist Alehall. Now, the humans probably think "Eh, a tavern" and don't think any more of it. With a culture that values alcohol, particularly ales and lagers, however, this may be a great honor. How many non-dwarves are even afforded such an opportunity? This is a grandiose display of respect, and the humans brush it off as nothing but an idle waste of time. The diplomat escorting the humans grits his teeth but leads the way for the uncaring humans.
Everyone arrives at the Alehall. There are a few dozen dwarves there, sitting at stone tables and drinking heartily, while laughing and talking as friends. It's a lively place, full of companionship and cheer. The mood dampens slightly as the humans walk in, and perhaps a joke or two gets interrupted. "So a human and an elf walk into an alehall, and.. uh... I'll tell you later." Anyway, the people recognize the diplomat and know that the visit is for the purpose of impressing visitors, and so return to carrying on, in a perhaps somewhat subdued sense, with a focus on looking like good people and wealthy enough to justify trade. The diplomat leads the party to the bar, and the owner of the hall comes over. Not just a barkeep; these visitors are too important. The owner warmly greets everyone, and reaches for five mugs, each easily able to contain a full quart. "Aye, welcome, 'umans! Fer sech honored guests, I'm wellin' to give you a q'ert on the house!" Apparently he's got a touch of an accent, and a more home-ish and rambunctious personality than the diplomat. Not all dwarves are the same person, after all.
Then, the party monk, somehow slipping in from the days before 4e, waves his hand dismissively. "Thank you, kind dwarf, but I do not drink alcohol."
An innocuous comment, to be certain. Polite as well, even overly so for most human taverns. In such places, the barkeep would likely laugh amusedly and fetch a water or milk for the man of temperance. The player of the monk is roleplaying his character, which is a good thing, and the other players probably see no issue with him refusing a drink. It's never been a problem before.
However, this time, the noise in the room quiets noticeably, with many of the closer dwarves looking at the monk in half shock and half annoyance. The gregarious smile immediately drops from the alehouse owner's face, as he pauses mid-turn. The diplomat coughs pointedly in the sudden quiet, and speaks with a certain emphasis, hoping to allay this faux pas.
"Honored visitor, you are in a dwarven alehouse, and the owner has offered you a quart. Perhaps you should accept his generosity."
This is how I think such a visit should be. Just being gruff and beer-fond humans isn't enough. Dwarven culture is a different culture. Just like eating a pork chop with your left hand would be offensive in Tehran, refusing a gift, particularly a gift of the hall's finest ale, would be a grave error in judgment in the dwarven halls. If you hitch hike by sticking your thumb up and out in some countries, you are likely to get attacked for giving an obscene gesture. If you angrily throw your fist into the crook of your elbow and show your middle finger in Tokyo, you will get little but confused looks. In a dwarven city, offering your host a gift of wine and bread might be considered strange. However, if you arrived and presented your host with a finely carved granite coffer, inlaid with a sparkling ruby and etched with gold, he would be overjoyed and consider you the very pinnacle of what humanity had to offer. It doesn't matter how fine the wine was, or how fresh and warm the bread was. It just wasn't the right gift.
So spend some time thinking about your dwarves. Think about what they are genetically and culturally predisposed to thinking is proper. Think about what they like, what they dislike, and what they could frankly care less about. Think about the differences in human and dwarven etiquette. Drinking on the job in human lands is generally enough to get you fired. In dwarven culture, it's absolutely the norm. An axe in human culture is just a tool, either for hewing logs or raiders. To a dwarf, and fine axe is worth more than all the elven wine and human paintings in the world. Nothing quite matches the beauty of that shining adamantine weapon, with the line of glittering emeralds down the haft, and the goblin-leather wrapping on the hold. It's an astonishingly valuable treasure. A human adventurer would look at it, shrug, and say something like "I use swords, not axes."
So yes, when you voice a dwarven NPC, feel free to play Gimli. Enjoy your beer, but also make a point to say and do things that are slightly outside the norm, because you aren't just a short human, you are a dwarf, proud and true.