Write how people talk. Two goofball slackers in their Twenties will talk differently than two devout nuns in their Fifties. In real life a clever person will say clever things. A dull person will not. In real life a clever person will be consistently clever or witty as he goes through each day. In your screenplay, most of the time we see him, he should be the clever, witty guy -- if that's the character's dramatic purpose. If you're writing the character for comic relief or to be a buddy/confidant to your protagonist, his job is to say or do funny things. We don't necessarily need to see the dark, sad or unhappy aspects of his life. Sitcoms are good examples of this.
Characters should speak in what appears to be their natural, everyday language, but they must avoid the repetition and digression of ordinary conversation. What they say must be carefully designed to move the story forward.
Characters in every form of fiction tend to be drawn from real life. Your success as a screenwriter will depend on your powers of observation and on your ability to portray what you observe. Characters are also drawn from what you read and what you hear.
Astute, observant screenwriters note the peculiarities, eccentricities and the special qualities of the people around you. Sometimes you'll create a character accurately drawn from a single living model. More often you'll use combinations of personalities that have moved or intrigued you to create a totally original character.
