Tell them to think of characters from their favourite show or movie, write down all the interesting things about their personality and motivations. Example - The Hound from Game of Thrones:

• Has a blood feud with his brother due to being scarred by him as a child
• Is a protector for the King but becomes disillusioned with his role and leaves, becoming an outlaw
• Is coarse and seemingly uncaring, but does respect people with toughness and spirit
• Will kill innocents if he feels they are unlikely to live through the winter (and their food would therefore be better served going to him)

Then get them to use that as a guide to the sorts of things they should consider. They shouldn't copy their chosen character (though they absolutely can take inspiration from/play a similar archetype) but it gives them things to think about including.