Well, yes, Voice Over is also used for narration, since narration is also recorded and dubbed onto the soundtrack. Same character extension, different uses.
INT. HOTEL ROOM - NIGHT
Derek lies on the bed, blah blah.
A KNOCK at the door.
DEREK
Who is it?
PLOT DEVICE (O.S.)
It's me.
DEREK
Go away!
More KNOCKING. Derek ignores it. The phone RINGS. Derek sighs and picks up.
DEREK
Hello?
PLOT DEVICE (V.O.)
Open the door or I'll shoot the lock.
Derek hangs up, goes to the door.
DEREK
I bet you don't even have a gun!
BLAM! Terrified, Derek stumbles back and falls onto the bed.
The door swings open, Plot Device enters, smoking gun in one hand, cell phone in the other.
PLOT DEVICE
You'd lose that bet.
PLOT DEVICE (V.O.)
There was a lot this guy didn't
know about me. Tonight was
going to be a real eye-opener.
-Derek