A few years ago a fellow writer told me he had difficulty writing dialog saying it was almost foreign compared to narrative prose. I gave myself as an example and advised he try writing pure dialog without any narrative prose only keeping what could be fit inside quotation marks. The suggestion came with the classic note to write each character’s lines in different colors to help differentiate them and all together sounded like a regurgitation of advice I’d read which worked for a chapter or so for me before I moved on from it. It wasn’t bad advice, I’m sure it’s helped many writers, but I now feel it too specific to have told him that without ever reading his work. If I could redo that conversation, I’d rather have said something like this:
When you write any two or more people talking to one another just remember they’re all their own person. One of them wants to save the world and be a hero, one wants to find love, the other’s looking to invent a device that’ll let humans walk up walls. It’s good advice for writing a character and what reason is there to leave that out when that character opens their mouth?
It’s less complicated to do this for the protagonist because you easily identify with them and likely want the same things they want. You create a backstory filled with joy and hope only for it to be interrupted by a dangerous challenge which allows them to pursue their dreams if they chose to accept. Much of those joys and hopes and the big, beautiful dream are things you either experienced or wish you had. But, when you add another character into the mix where you don’t possess that same innate motive they do the words don’t come out of their mouth naturally. Unless you have twin protagonists only ever talking to one another there’s going to be a need to write dialog from another person’s view.
It’s common to patch this issue by making every morally good character fall in line with the protagonist. If the protagonist is trying to stop the evil lord and save the world then so will all his party-mates. Then everything everyone does is the same and you don’t have to think about them as individuals, I mean everyone’s a good guy. Maybe the girl in the party will want the hero too, sure. But this will make every character sound robotically alike chasing after the same end. Even if they arrive at that solution from different backgrounds, which they should; even if they have different personalities, which they should; even when they disagree about how best to achieve their goal; if there’s anything good in this world; they can’t start every conversation with, “Gee, I sure hope we overthrow the evil lord and save the world today! How about you?” You have to think about what these people want at the end of their journey after they’ve overthrown the evil lord. Does the protagonist want to be king and piece the world back together? Does the magician want to found a college to find a pupil to surpass her skills? Does the brute want to open a ramen stall? Even if they don’t actively know it or bring it up ever, you should keep that in the back of your mind when writing their dialog. How does what they want shape what they’ll say to get what they want? Separate what they’ll accomplish for the plot from what they’ll achieve for their own sakes i.e., in the epilog after the world is saved, how is everyone living their best life?
The other thing, maybe you’ve seen this, is underwriting the secondary characters. Say the hero needs to buy a shield from a shopkeeper to start his quest but he can’t pay what the shopkeeper wants for it. As much as we know the hero needs that shield and as much as he protests to the fact, the shopkeeper says no, he needs the money. If you leave it at that he’s going to appear like a callus, mustache twirling shekel pincher. There must be obstacles for the protagonist to overcome but not everyone and everything can be actively opposing him for nefarious reasons. It’s unrealistic and runs out of steam too fast to sustain a novel length story. What’s more reasonable from the shopkeeper’s perspective? “Your adventure be damned—I need money!” Or, “I’ve got a family I need to support. I can’t sell something for less than what I bought it for. I’ve been doing this for six years now, you’re not the first person to say all this to me. I don’t want to see my children starve in front of me. Do you have the money yes or no?” He doesn’t and probably shouldn’t say that out loud but the weight of his actions combined with an exhausted sense of treading water should be the glum mood the shopkeeper gives off while talking with the protagonist.
More than the shopkeeper’s words, if he has that concern in the back of his mind he’s going to act differently too. If you’re more comfortable with narration then remember the art of non-verbal communication. Don’t constrain your mind that dialog is just what’s between quotation marks. If you say you have difficulty with dialog then you likely have better versatility with prose and describing how people feel. Then, all you need to transfer that talent to bolster your dialog is cut a few non-essential words and replace them with non-verbal descriptors. Rather than a no as a refusal have the shopkeeper’s eyes narrow as if bored to near sleep by the protagonist’s repeated pleas. If you’re good at writing a facial expression then garnish those in. Small but specific gestures can reveal a lot about secondary characters and imply a deeper cause to their actions. A shrug is a shrug but a shrug paired with an eyebrow that rises halfway up the forehead and scrunches wrinkles like the sand waves in the Sahara conveys much more. Bring the same confidence of your prose into your dialog and see if you can naturally avoid any sense of defeatism.
It takes time to draw up these characters’ motivations and it’s tough to devote that much effort to a character who’s introduced and left behind in a single scene or to characters who are flat and static throughout the story. You want that effort going to the important people you and the audience will identify with, it’s understandable. My counterthought is like a good side of potatoes and vegetables to a steak, good secondary characters can boost a story to greater heights. Once you have the character’s motivation in mind you should whip up their life in broad strokes up to the moment they are introduced into the plot and talking in the present scene. What they’re doing in the present isn’t the only act they’ve undertaken to achieve their goal but it is a small piece along the way. You know their past. Think how will this give them the future they want for themselves and others and write their dialog with that in mind.