1. A method for passive training for automatic speech recognition, the method comprising: maintaining both a first model and a second model that are configured to detect a spoken keyword or a key phrase in spoken utterances, wherein the spoken utterances comprise words and/or phrases spoken by a user; utilizing only the first model, the first model being a speaker-independent model, to detect the spoken keyword or the key phrase in spoken utterances until the second model has been trained; passively training the second model, using the spoken utterances comprising words and/or phrases spoken by the user, to detect the spoken keyword or the key phrase in the spoken utterances; and in response to completion of the passive training, wherein completion requires performing the passive training with a plurality of the spoken utterances satisfying a predetermined set of criteria, switching from utilizing only the first, speaker-independent model to utilizing only the second, passively trained model to detect the spoken keyword or the key phrase in the spoken utterances, wherein passively training the second model includes selecting at least one utterance from the spoken utterances comprising words and/or phrases spoken by the user according to at least one predetermined criterion, and wherein the at least one pre-determined criterion includes one or more of a determination that a signal-to-noise ratio level of the selected utterance is below a pre-determined level and a determination that a duration of the selected utterance is below a pre-determined time period.