I would assert that a principle is a belief which guides behavior, yes, but with the understanding that the weight of the guidance and the weight of the conviction varies.

I don't mean that in a fully negative way, since belief and choices are rarely atomic.

Take, for example, someone who believes animals shouldn't suffer unnecessarily. That can manifest anywhere from veganism to just avoiding factory farmed meat. I wouldn't point at any one position on that spectrum and say they don't believe their own stated principle, but I would say that some have weaker convictions than others.