If you want to be a good actor, you must be willing to invest in your training and that does not only cover - but also - the academic training, but also learn other disciplines, read a lot and seek information on various subjects.
The issue may give rise to the question whether the good actor is born or made, but even starting from the idea that the person is born with a natural capacity for acting, it is important to remember that famous phrase that sentences that talent is composed of 10% inspiration and 90% perspiration. And that's it, so it's no use having skills if you do not work on them, increasing the potential. In principle, a fundamental characteristic of any actor must be determination; This is because this is a difficult profession, in which one is a potential unemployed, since it is not a stable occupation, most of the time. The history of cinema and television is full of examples of great actors who were discovered quite late, because they had no luck in the castings. Therefore, you have to be prepared to receive negative and not drop your arms for it. 
Training is a key point, and although there are cases of actors who have not gone through a school, it is desirable that they study to enrich themselves with methods and techniques and equip themselves with useful tools for the art of representation. In addition to continuing the acting career, it is very positive to take short courses and attend workshops, especially if they are dictated by prestigious artists and recognized ability.
Apart from the techniques of action, the aspiring actor must aim to have a comprehensive education, so it is invaluable to learn to dance and sing, for example. In addition, these are skills that can even be used to win a role. To the extent that the actor is trained in various disciplines, he learns to extract the best of his physical and mental possibilities, being able to perform a better interpretation work. 
But training is not just about learning techniques, but also about investing in knowledge. And here we do not speak only of knowledge directly related to the profession: all types of knowledge and learning are valid, because the more information and data one has in mind, the greater will be its expressive and creative capacities. Therefore, it seeks to read a lot: books, magazines, newspapers, websites. Try to always be in an insatiable search for knowledge.
Besides the rational side, cultivate your sensitive side. And is that a good actor, by nature, must have sensitivity to capture and absorb what is around him and translate it into a solid and credible acting work. Apassionate: you must have passion for what you do. It does not matter if it is a large or small paper, whether it is a large production or a short amateur. You must fall in love with your character, give everything of you in the performance and do your best work possible. That will print another nuance on the screen or on stage; and believe that it will be noticed. Another important point is to be humble. You can be a great actor, but if you do not have humility you will end up stalling, believing that you are "too good" or that you have nothing left to learn. Listen a lot, both teachers, directors or colleagues. All of them can have interesting things to teach you. Do not act like a star, but as an artist always willing to learn.
You should also immerse yourself in everything that surrounds your character. If it's a doctor, it's good to talk with one, visit a hospital, read about related topics and, mainly, observe. In fact, observation is fundamental in the actor's work. Also, make sure you talk to the director about what you are looking for in the character and also observe all the work that is done behind the scenes or behind the scenes.
Finally, always try to be punctual at the time of the rehearsals and at the time of filming or staging. Try to arrive before the scheduled time, take the opportunity to chat with the production people and show yourself always willing to repeat the scene or make a new review of the text, no matter how tired you are. That will take you to be a better actor and to perfect your work, turning it from good to excellent. 