I would personally develop my interests through a mix of research and exposure. As others have mentioned, keeping up with trends and learning more about practice areas through research is a great way of showing and developing your interest. I also found that Forage programmes were a great way of gaining exposure to the type of work that trainees would actually complete in those practice areas - for example, when I was uncertain what practice areas interested me, I completed a few programmes which helped to give me an insight.
It's very normal for interests to evolve or overlap - I think the key thing with commercial law (for most firms, though not all!) is to keep an open mind to a variety of practice areas and remain curious, even if you hadn't initially thought you had an interest