Pronouns are often dropped in Spanish when the subject can be clearly inferred from the sentence. In this case, "Es un excelente actor" ... it is could be assumed that "He" is the only one being referred to. There are two other possibilities here ... Ella (She) and Usted (formal You). If there is no one else being referred to in this context, then you can drop the pronoun. However, it is also not wrong to add the pronoun for clarity or emphasis.