Writing dialog for a character can be easy and enjoyable. However, it can also lead to common mistakes that can make two people sound exactly the same. We recommend that you design your characters with traits that only they have. Perhaps they come from an area of the world where people speak with an accent or use slang that the other characters done. Characters will often have catchphrases that make them different than the others.
Remember that characters are different people. They have unique desires that will fuel their desires and their fears. Using dialog to choose the right words that will symbolize these vectors will subconsciously let the audience know that these are indeed not clones, but interesting people that make the film as entertaining as possible.
Also remember that some people talk a lot while others barely mutter a word unless they have something meaningful to say. Your script should be very entertaining when just reading the dialog. One should not get lost just reading the dialog. Properly written, your dialog will motivate the reader to want to read any action that surrounding the spoken word.
Do Your Research
If you are writing a character that originates from a part of the world that you know very little about, use every resource at your disposal to get informed. Videos online can be a great resource, but nothing replaces visiting that region of the world or at a minimum, having extended conversations with these people in person. You will find that stereotypes may make an uneducated audience believe your character, but those who are from that area of the world will laugh at your fake attempt to mirror their culture.