How To Create Memorable NPCs

Non-player characters aren't just cannon fodder or set pieces for players to destroy. NPCs can be utilized for any narrative purpose or game play needs. NPCs will usually be the ones to create conflict, which is necessary for a good story.

Prepare a characters emotion or personality type.
Describe the characters main emotion in one word. Descriptors can be really simple. Nervous, confident, joyful, grumpy, sad, aloof, indifferent, etc. Once you have a specific word to describe an NPC you can base your entire performance on that one word. If you would rather improvise during the game, you can prepare a list of random descriptors and personalities to have on hand. Also you can prepare a list of random names prepared because we always need to make a new NPC during the game.

How does your character deal?
Another way to describe your characters is how they operate in a deal. Are they givers, takers, or matchers?

A giver would offer whatever they can to help and support his friends without expecting anything in return. A giver is happy to work as a team without getting credit for himself specifically.
A taker will manipulate and lie to get whatever they want. A taker will always steal credit for achievements and always keep the glory for himself.
A matcher is someone who will always repay a favor for a favor. If someone does a favor for a matcher, he will feel like he owes them a favor. I got this idea from reading Give and Take by Adam Grant.

Set up a goal. 
NPC goals can be really simple too. A farmer wants a new plow that works better. A blacksmith who is in love with the prince. A neighbor keeps trying to borrow a cup of sugar. An apprentice who doesn't want to learn her trade. If you keep NPC goals simple, your world will be grounded realistically and feel more believable to your players. Not all their goals need to be about avenging their father or finding a secret family heirloom.

With these goals set up, you'll be able to direct the conversation for a good role play encounter.

Now that you have a personality, or emotion word, a giver or take action and a goal, you can use all three together to tell you what actions the characters will take. 

Let Your Characters grow
Allow your characters grow throughout the game itself. Improvise, learn, and adapt your characters just like a player character. Find their voice during game play. Be patient with yourself and your characters, let them grow naturally over time. The best characteristics are revealed in game.

 Don't forget to write down important things that NPCs say or want. This way your world and characters will be consistent from session to session.

Don't worry about making NPC stats unless you know specifically that they will be fighting the players. If the NPC never sees any combat, you won't need their stats. You can make up or fudge any of their rolls if you need to.

Be the character, not a game master.
The Angry GM says "Play the character instead of running the game." 

While in conversation with players, don't trying to narrate and manage the game, just act in character and react to what the players do. Don't think about what will happen next in the game until the conversation is over. 

You can even narrate with an NPCs voice. Focus solely on how the NPC will react to what the players are doing. Focus on your NPCs main goal and try to control the conversation in character. practice reacting how that character would react to different things.

How Helpful are your NPCs?
Use NPCs as the voice of reason instead of the DM talking. When giving out quests, use NPCs to give players hints about what monsters they might fight or warnings about what is ahead. Hopefully, (for their sake) your players are smart enough to ask questions.

Set up characters that the players can go to for advice or help. A wizard who can identify items, a cleric who can heal, and criminal contacts. Maybe there is a woodsman or ranger who can guide the players through the forest or a hireling who has experience fighting specific monsters.

NPC Voices.
Along with accents, you can also consider talking just by changing your voice a little bit. Make a NPC talk a lot faster than your normal voice, or slower. Talk with a deeper or higher voice. These changes are simple and effective. Every time my voice gets lower, my players know that the bear is talking.

You can also use different vocabulary for different types of people. Perhaps your players encounter a well-educated higher class scholar or wizard. They would use fancier words and some magical technobabble. A bum on the street or a barbarian raised in the forest probably won't be using big words.

If you want to get better with accents, you'll have to practice talking a lot in that voice, outside of the game.