I used to ask people how to explode a slice on a pie graph, or how to make a pie graph 3D ..... depending on the individual.
The best way to approach an interview is be honest about your abilities and know the product/s as best you can.
In an interview with Tableau or an interview regarding your skills with Tableau?
Depends what role you're going for at a company, if you're in analytics and reporting they may ask you a few questions on how you currently use the tool. Some technical questions may be asked, e.g. explaining LODs, parameters etc.