It depends on school. Actually, it depends more on the person interviewing you and how your conversation goes. At some interviews I have talked for 30 min- 1 hour about research I did and research done at their d school. At others, it wasn't even mentioned. Not one particular school will for sure have research experience related questions on interview, it really does depend on where the conversation goes. That said, you can direct the conversation towards talking about research if you want to.
To answer whether interviewers were tough on research qs, I'd say again it depends on teh interviewer. At a particular school I was asked to give a 5-10 minute presentation about my research project and write stuff on the whiteboard. The interviewer asked me questions about the project later.
I would say publications influence admission probability slightly. On the same level as other volunteering/shadowing/community service experiences.