Take the time to get to know your characters, and it can pay dividends when you starting writing your story. A character sketch can be an excellent tool for doing that. You should definitely do one for each central character and at least a shorter version of minor characters.
The place to start is with the character’s appearance. First, work out the basics–age, race, eye and hair color, height and weight, skin tone. Then go on to how your character dresses. Keep in mind that the character may dress differently in different situations. Lee Child’s character, Jack Reacher, is distinguished by his practice of buying new clothes when he needs clean clothing and throwing away the dirty ones. Finally, consider the unique characteristics and mannerisms of the character.
…distinctive attributes make a character memorable.
It’s here you can conjure distinctive attributes that help make a character memorable. Think of John Sanford’s Lucas Davenport with a thin scar running through his eyebrow to his cheek, and a smile that can be scary. Then there is Lieutenant Dave Robicheaux, a creation of James Lee Burke, who has ink-black hair and a mustache except for a white patch above one ear, a legacy of malnourishment during his troubled childhood.
Another thing that can be helpful is to look around online and find a picture of someone who matches your vision of the character you are trying to create, and include it in your character sketch. I am a visual person, so if I describe the character in a book or story, the photo helps to do it in a more creative way than just listing vital statistics.
Below is an example of the appearance section of a character sketch for a book I am currently rewriting. The photo is one that I found browsing the internet that fit the bill of what I needed.