Fictional characters can be said to be more like musical instruments than real people. They play a role that adds to the overall atmosphere of the piece. They make their own unique sounds, possess their own timbre, and have their own specific range of notes. But they can also fall out of tune or become discordant with the rest of the instruments, or the piece as a whole.
Characters need to be true to their nature, play their role with verisimilitude, and add value to the piece. But they need to be believable. Their actions and words need to adhere to a set of rules a reader might expect to accept. So, yes, they can wield a wand or a light saber, but their dueling banter must still come off as genuine. Their motivations ought to be human, even if their species is not, unless there are well-described reasons for them to act differently. Do they askew romantic attraction, but it turns out they are compelled to find a mate every seven years? Do they grow back a new head if a secret agent shoots the old one off? And if so, then why is this important to the story?
It’s good to be imaginative. But it’s important to be plausible. And consistent. Keep notes, or use a spreadsheet to document idiosyncrasies of your characters, no matter how human or alien.
And listen to real people talking to each other. Go to the mall, or sit on a bench at a popular park, or at the beach, or anywhere people go and chat with each other. Listen. Take notes. What are they saying? What do you imagine they are actually hearing? How long do they focus on the Green Bay Packers before they start talking about baking fails or carburetors or medical procedures?
The more you study human conversation and discern the reasons people make choices, the more plausible your characters will be.