Differentiating characters is a problem we all have. Your idea of varying sentence structure is a good one. An old professor might talk in long rambling sentences. A student might talk in short, clipped sentences full of slang.
Here are more ways you can may people sound different.
- Give the character a unique expression or exclamation that they always use. It could be as simple as “Heavens” or “Huh?” The Urban dictionary can provide ideas for younger characters.
- Make sure the character uses words related to their occupation, history, passions, or interest. A fisherman will refer to the sea. A language professor will refer to particular books or writers, and so on, especially in metaphor and simile as described in today’s lesson.
- Give the character a habit that is unique to them such rubbing their neck , pulling on a cuff, or scuffing their feet. An overused one is a woman, when stressed, touching a special pendant she wears. In dialogue, show that action in your action tags.
- Also in dialogue have action tags showing the character fiddling with their clothing or belonging. This will help differentiate them.
- Give the character a cliche they repeatedly use in different ways like “greased lightning” or “slippery as a fish”
Let me know if any of these help.