I might be doing it wrong but it might also be that I'm on a Nexus 4. When I do that it brings me to Google search but the voice prompt still needs to be activa

I might be doing it wrong but it might also be that I'm on a Nexus 4. When I do that it brings me to Google search but the voice prompt still needs to be activated via tapping the mic button or using the voice command. Is there a way to change that to just immediately activate it? And what about from when the screen is off? I have a software home button so can't use it until the screen is turned on unfortunately.