Do All Your Characters Talk The Same?
If you want a good idea of how to not write characters, go read Robert Heinlein's Number of the Beast. I actually like most Heinlein I've read, but I'm pretty convinced that all of his later books are just horrid. In Number of the Beast, the most positive thing I've seen Heinlein apologists write about the book is that perhaps it was utterly terrible on purpose, as a way of teaching people how to write well. I guess that's possible, but I'm hesitant to believe it, when I don't think Heinlein ever said as much on the record. It seems more likely that it's just bad, but just random enough that if you really squint, you could choose to see an elaborate scheme that justifies how terrible the book is. I just don't buy it. To get to the point, allow me to relate the characters to you: Robert Heinlein and of course OK, technically in the book, they are a guy in his late 20s, a woman in her early 20s, an older male professor, and an older female socialite, but they all have the same voice.
Related: 2. characters