all talk (and no action)
Much discussion but no action or results, as in Don't count on Mary's helpshe's all talk, or Dave has been saying for months that he'll get a summer job, but he's all talk and no action. This idiom may have begun life as all talk and no cider, which Washington Irving cited as an American proverb in Salmagundi (1807). However, similar sayings antedate it by many yearsfor example, "The greatest talkers are always the least doers" (John Ray, English Proverbs, 1670).
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.