If you manage your tasks with Google Now, you have now more options to open Google Now even from the lock screen of your Android device. The "OK Google" detection feature now works from any screen, and you just need to do as follows to be able to call Google voice search (Google Now) from any screen including the lock screen:
1. Open Google app (Google Now / Google Search) and make sure you have the latest version installed
2. Tap the 3dot icon and then go to Settings.
3. Go to Voice settings
4. Tap the "Ok Google" Detection option
5. Make sure to mark both "From any screen" and "From lock screen" to let the Google Now respond to your voice when the screen is on, the device is charging, or it is locked.