Informal, First part consists mainly technical questions about ontologies, epistemologies and preferred methods. Coding skills are not always required. Second part is supervisor-specific, mainly linked to knowledge about the professor you want to work with

The correspondence starts by an email exchange first, and then it is usually followed by an arranged skype chat with prospective supervisor. And then there is usually further email correspondence to discuss research topics etc. After that the candidate may be asked to make a presentation to the research group he or she is planning to join. The presentation is fairly informal and you get some questions from the attendees. Finally the candidate will discuss financial situation and the expected duration of the candidacy, and some preliminary research plan.

Had the chance to meet the professor for whom I worked during an internship of my MSc. It's always a good idea to know the Prof before actually applying. Once he said that he wanted me to stay, we discussed the research topic in more detail and submitted a formal application through McGill website.