I have been asked this question by my colleagues (residents and physicians) several times. I would like to transmit this question to the experts like you to help them to get the appropriate answer.
To properly be called a researcher, the minimum skills are: writing and publishing. In order to do this, it is compulsory to read and reflect. These, I believe, are the minimal requirements.
An observation time (6 month to a year, where they set up for instance a review) is very helpful, because it is very difficult to predict who will be succesful. Intelligence, reflection and a will to succeed are qualities you need always. But although research seems to be for the lonely wolf, being a team player is the overarching component. In contradiction to a clinical observation time, after an observation time in research, it is easier to predict who will be a good investigator but also a successful resident.