Identify what it is about your process that makes you lose interest in the project. There isn’t really a one-size-fits-all answer to this question, but here are some questions for you to consider: 

What about the project sparked your excitement in the first place? Why does it no longer excite you? 

How far into the project do you get before losing interest? 

Did you plot too much which has sucked the joy out of writing?

Did you plot so little that you get frustrated and give up? 

Did you just get Shiny Idea Syndrome and want to work on something new?

At the end of the day, you will need to decide for yourself how important is it for you to have a finished work and commit. Figure out what it is that makes you lose interest, and come up with a solution to combat it. 

… do you mean characters? i’m assuming you mean characters.

some of it is in context, not the characters’ actual speech. you can comment in narration how someone’s voice is rich and deep or light and tinkling or rough and loud or soft and soothing, etc. etc. it helps form specific voices in the readers’ heads. i mean, yeah, anyone’s voice can be any of those things at different times but some people’s voices are always soft, or always tinkling, or always rough.

in speech, people have quirks. some people stammer more than others (”do you— er, would you— i mean— could i trouble you for…?”). some have more tendency than others to repeat themselves. everyone has a few words or phrases they say too often, usually unintentionally (a few of my own are “awful” and “kind of” and “oh my god”). most people have a few catchphrases, which are said intentionally (like ”thats right, folks” or “serves you right”). some people have greater tendency to ramble, some put in a lot of unnecessary words and explanations (”there was this one time when i went out to this house— it was my friend’s house, who i met in high school, we’ve been friends for a very long time—but yeah, so when i went to his house…”). some are concise (”once, i went to an old friend’s house and…”). some like to swear (”aw, fuck, man, let me tell you about this one goddamned house”). some are eloquent (”what an atrocity” versus “this is terrible”).