Now you can use ‘OK Google’ hotword to do things like search, get directions, and send messages from any screen including your lockscreen on your Android phone. Google has updated the Search app (v3.5.14) for Android by including the nifty feature ‘OK Google’ voice detection from any screen.
Once you updated the Search app, you have to enable the option by going to Google Now Settings -> Voice -> Ok Google Detection and enable “From any screen” and “From lock screen”. Once you have done the settings, you need to say “OK Google” hotword three times to recognize your voice.
Please note that, by simply installing or updating the Search app will not enable this feature for your account, because this feature is rolling out by account. In addition, this feature only works if you set your voice detection language to English (US).
It appears that, some users who have already got this feature are able to use voice recognition even when the screen is off (while the phone is charging).
It will take some time to get the update for your device. However, if you do not want to wait for the update, you can download the APK and install it just like any other app., though it will not necessarily enable the feature for you.
Head over to Androidpolice to download the app.