First, a phone or video conference to gauge your interest. Second, an invitation to present a seminar on-site (unless you don't live in the country). You will spend a day meeting various groups that will try to figure out where you might best fit. Questions asked will vary wildly depending on which group you meet. But they restrict the questions to what you claim to know in your CV. Ad-hoc puzzles or tricky questions are rare, but your day will end with a written Perl/R quiz that usually sticks to the basics.