Many players and DMs love roleplay. It's at least half the game of DnD, sometimes more if your table is a big fan of doing whacky voices and writing down unnecessary names. But, just because people enjoy roleplay doesn't make it an excuse to stop improving. Here are some ways you can begin to take your roleplay to the next level.
1. Act and Speak Irrationally
Most of us are logical, ration people. We do things that makes sense. But, in reality, and especially in fantasy, not everyone is ration or logical. Some of the most compelling people and characters are those who go against the grain. A merchant that refuses to take currency, a town guard that refuses to use their weapons, a baker that's jealous of the bread they sell. These things don't make sense, but they make for interesting interactions.
2. Have Your NPCs Experience Change
Is your party in town to kill the king? Rescue a lost child? Get rid of a terrifying creature? Well, simply ask yourself how that effects the people around them. All of these things would and should have consequences on those in the community. An NPC would reasonably get much happier and more confident once they know the horrible beast have been slain. And never forget that the NPCs shouldn't forget about how the party treats them, and react in kind.
3. Lie
NPCs have just as much to hide as any given PC. People tell little white lies all the time, they hide the weirder stuff about themselves, their towns and their businesses. They barter, they gamble, the exploit each other for greedy ends. Not all your NPCs should tell the players where to go out of the goodness of their hearts, sometimes they'll direct them into a thieves guild trap for a cut of the shakedown.
4. Accents
There is absolutely no need to put on voices for a great DnD game. But, in my own personal experience I've found that when you throw on an accent your players will love it. An additional dimension to this, and one that makes it even more fun and contiguous in the world, is having regional accents. If everyone from a city speaks with the same accent, and then your players run into someone with that accent outside the city, then they'll know where that NPC is from and have fun with a callback.
5. Education
The vast majority of commoners in DnD are uneducated, which means they shouldn't be able to read or write, and probably will have little to no knowledge of the world beyond a few miles. This provides ample space to play around in, when your players ask NPCs about books or signposts that they can't read, or give directions to a city that they've never actually been to or seen on a map. There's nothing wrong with a dumb NPC. Also, many dumb people consider themselves incredibly smart and knowledgeable. Just something to keep in mind.
6. Preplan Cool Dialogue
Often this refers to boss dialogue. If you have something cool you want a boss or NPC to drop on your players, just workshop it a little earlier and write down your best option. This lets you focus on whatever's going on in the moment, without forgetting whatever it was you wanted to get across. This also applies if you're trying to mislead a party, because players sometimes take different meanings from interactions than you were expecting. So workshop your stuff, and stick with the best of what you got.
It's never the wrong call to add some character development to your games, be it for PCs or NPCs. Watching as characters grow and change, and keeping interactions going between your NPCs, bosses and PCs is a joy. It's one of the best parts of DnD. Lean into what's fun, listen to your players, and everyone will have a good time. Also buy our dice, it'll help your roleplay too for sure. Don't ask how.