A person who only talks and does nothing is often referred to as a "big talker" or "all talk and no action." This term implies that the individual is proficient at making promises, boasting, or expressing ideas verbally but fails to follow through with any concrete actions to back up their words.
Another common term used to describe such a person is a "blowhard." This term is often used to describe someone who talks loudly, boastfully, or arrogantly but lacks substance or accomplishments to support their claims.
In colloquial language, you might also hear phrases like "hot air," "empty talker," or "full of hot air" used to convey a similar idea – someone who talks a lot but doesn't take meaningful action. These descriptions generally carry a negative connotation, as they suggest that the person is not reliable, trustworthy, or capable of backing up their words with deeds.