On the topic of "when someone shows you who they are, believe them". To me, it makes perfect sense. People are what they do, not what they claim to be, want to be, or say they are.
If someone constantly tells me that they care about me but their actions say otherwise, then I should believe their actions, not their words.
And yes, it is usually used in a negative context, because oftentimes people are in denial about who their partner/parent/child really is. Because people can be charming with their words, but their actions are what shows us who they really are.