Each thesis supervisor and doctoral school has its own selection criteria. What you need to remember is that the most critical requirement is your capacity to be able to carry out research work. This requires being independent, because since there are no or not many classes in PhD/doctorate, you will have to work in a lab on a specific project. You will be assessed on your capacity to know where to find information by yourself (bibliography) and to be able to develop and implement a research protocol. If you have already published or presented a poster in a conference, it is an important point in your resume. If you have done a Masters' internship in a laboratory, this is also a good thing.