I'm looking for a word which describes a person who just talks but would not do or practice what he/she says - like somebody who doesn't practice what he/she preaches.
From OED /big talker n. (a) a person who speaks boastfully or extravagantly, a person who ‘talks big’; (b) a person who is much given to talking, a great conversationalist.
1775 Gen. Election: Lett. between Friends I. xxiii. 235 These big talkers, as my brother observes, are generally the least doers./