One crafty technique that should be part of every writer’s toolbox is using foibles, quirks and mannerisms in your characterisation. You should do this for two very good reasons:

They individualise your characters. All humans have foibles, quirks and mannerisms. They’re the minor and unconscious ways we do things, from the way we walk to the way we talk to the way we eat our food. Allocate a handful of these to each of your characters in incidental description, and instantly they’re more realistic, more human, more convincing.
They can create a Moment of Recognition. As a writer, we’re all aiming to engage our readers. One of the most subtle – but most powerful – methods is when a reader recognises something in one of your characters that is like someone they’ve seen, or like someone they know or – best of all – just like themselves. Dropping in well detailed foibles, quirks and mannerisms is a useful way of providing opportunities for those Moments of Recognition that help your reader develop a deep and enduring engagement with your story.

For example, consider someone who cannot finish drinking something without adding a satisfied ‘Ahh!’. We all know, or have seen someone like that. If your character – major or minor – displays this foible/quirk/mannerism, it instantly individualises them (because this character is the only one in your story who displays this FQM) and creates a potential Moment of Recognition. Achievement unlocked.

As a writer, you should practise observing people, noticing these tiny aspects of our behaviour, then collect them and roll them out to make your characters more realistic, more individual and, with luck, create that wonderful moment of recognition hook for your readers.