One of our readers has asked about how to disable it. It seems Cortana can do much more now ( post GDR1 update ) when your phone is locked with or without a pin. If it is enabled in Speech settings, one can do a long press on the search button to launch Cortana when phone is locked.
Before Windows Phone 8.1 update 1 ( GDR1) it would just allow to initiate calls or control playing music. But after GDR1 update, it now supports “asking questions” and one can strike normal owner to Cortana conversations.
In case you are worried about someone misusing this Cortana feature to steal you data or breach your privacy, you can disable it by going to Settings——> Speech—–>Uncheck, use speech when phone is locked. Check the screenshot above.