When designing a character, I start with how they talk. This is a holdover from my days doing both tabletop and live-action roleplaying, where 90-95% of what you will do
with the characters you create is talk. Generally my first step is what I call the "screen test"...I'll get a few moments to myself and actually try and act as my character, imagining the situations I plan on putting them in and spouting lines of dialogue I think they'll say. Just to see what the character is going to sound like. Again, its a holdover from my roleplaying days but it has served me well as a writer.
I've often maintained that a good test of dialogue is to say it out loud. If it doesn't sound right, it probably isn't.
Now, I've said this before and a lot of people respond with various flavors of "well, we want characters to talk better than we do in real life
", and that is totally true. But dialogue at the least should sound good to the ear of the writer. If it doesn't, how does it stand a chance of sounding good to someone else?