Is it illegal to video record someone without there knowledge?

Just wondering because I bought a device, just for fun, I can wear it and what it is is a disguised video recorder with sound. From what I understand, people are recorded all the time and banks, stores, ect. so it seems like it would be ok. From what I understand, it's only not ok if it is sold or somehow broadcasted without the persons permission. Is this incorrect? I've actually recently come across a situation where I may be able to use it to help me in a law suit, my wife was fired for speaking up at work about how her boss had sent sexual text messages to her, her boss ofcousre did not like that so he fired her. She still has access to communicate with him so there may be a way to apply this device. This guy has a lot of money so hopefully God willing justice is served and we win this case. Also I live in tx.