Contextual filter by an argument that is not in the URL

Hello. I have a question: how can I display a list of all nodes similar to the current one based on a certain field in a block on the node page? For example, on a movie page, show all films by the same director? It's clear that I need to use a contextual filter in Views. The question is: where do I get the argument in this case? If it's only through PHP code, what would be the template for that code? I understand it might just be a couple of lines, but I'm not familiar with programming, and all the solutions I've found online don't work.