all talk (and no action)
Much discussion but no action or results, as in Don't count on Mary's help
she'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).