Introduce as few characters as possible in the first scenes of your screenplay and make sure one of those couple of characters is the Hero of your story. Your audience will connect with the characters you introduce at the beginning and they will believe one of them is your hero. If your hero is not amongst those first characters you will confuse your audience. The other option is to introduce your Villain in the open scenes. This was done in each of the original Star Wars films where the audience was introduced to Darth Vader (and then later, the Emperor) in the opening scenes. Don’t waste your opening on characters who mean nothing to your screenplay, you’ll loose the audience right away.