When hiring a person for a technical position (including a post-doc), any employer will want to have some confidence in the degree of technical proficiency of the applicant.
If the PI already knows you (e.g., through interactions at conferences), then they might not feel a need to ask technical questions. Likewise, if the PI has a strong relationship with your advisor and is willing to take a recommendation on trust, they might not ask technical questions. In pretty much every other case, you should expect some degree of probing to determine your technical competence.
What form that will take, however, is completely unpredictable. Evaluating technical skill in a short interview is a notoriously difficult problem, and people have lots of different idiosyncratic approaches to the issue. As a result, I would advise the following in terms of preparation:
- Do not treat this like cramming for an exam. Do not do any study or review of core technical materials. Either you have the skills and competence that they are looking for or you do not.
- Make sure you are familiar with the work of the PI and their lab, and think carefully about how your skills and background fit with their current directions (though you might get surprised by the PI moving in a new direction).
- Make sure you are prepared to talk about your own work, your skills and background, and what you want to get out of this postdoc.
- Get a good night's sleep and do whatever else you need to do to relax and be in a good and confident place personally.
Finally, remember that the goal of this interview is not to get yourself a job, but rather for both you and the PI to figure out whether you and the job are a good fit for one another. Make sure that in the discussion you are also prepare to look for the things that will help you be satisfied with this position as well.