Creating unique characters is key to making them memorable, though individualising a character should be implemented with care, else they risk becoming more cartoonish than authentic. The more authentic a character feels, the more relatable they become, and the more relatable a character is, the more emotional engagement they’ll engender from a reader.
Authenticity can be achieved by using blatant or subtle methods – what counts is repetition and consistency. In dialogue, a character can have the habit of speaking in long sentences, using contractions (can’t, won’t, etc.), or employing a unique linguistic style:
‘‘Wicked masster!’ it hissed. ‘Wicked masster cheats us; cheats Sméagol, gollum. He musstn’t go that way. He musstn’t hurt Preciouss. Give it to Sméagol, yess, give it to us! Give it to uss!’ (The Return of the King, J. R. R. Tolkien)
In attitude, a character may have a tendency to be heroic, stubborn, or calmly evil:
“A census taker tried to quantify me once. I ate his liver with some fava beans and a big Amarone. Go back to school, little Starling.” Hannibal Lecter, polite to the last, did not give her his back. (Silence Of The Lambs, Thomas Harris)
In behaviour, a character may have a habit of chewing tobacco, spitting, or being an exceptionally hard worker:
‘Boxer would even come out at nights and work for an hour or two on his own by the light of the harvest moon. (Animal Farm, George Orwell)
Idiosyncratic habits, by themselves, are unlikely to be noticeable in a single scene; yet as the story unfolds, a reader will begin to recognise (consciously or otherwise) how specific characters behave, making them feel distinct, memorable and authentic.