How do I approach a postdoc interview when my background is not same as the professor's? My PhD research was applied mechanics. I used commercial finite element analysis software to simulate thermal spray process. My research did not involve developing any weak-form formulations or any new theoretical model. I just implemented existing models and the analysis was the novelty of my research. My publications are in applied journals like journal of thermal spray and technology.
Now the professor who's going to take my interview works in theoretical model and formulations development. His papers are in core mechanics journals. However, the advertised postdoc position description aligns with my research experience and I believe I can be successful there. But I cannot produce papers similar to the professor's profile.
How do I approach the interview in this case? Should I tell them that my research experience is applied and development of formulations is not my cup of tea?
My background is in computational mechanics, so I have some background to answer your question. It seems to me that you and the professor would make a good research team because your research interests and experience are complementary. Do not assume that the professor wants you to work on weak forms and model formulation. S/he might want to go in a more applied direction as @astronot said. Instead of assuming that the professor wants to produce more papers like s/he has done in the past, ask in the interview what you'd be expected to do.
Second, it might also be useful for you to learn weak forms, just like it might be useful for someone with a mathematical background to learn applied stuff using commercial packages.