An Extension is a technical indication about a character's Dialogue, placed to the right of the Character Name. For example, an indication that the dialogue happens off screen or as a voice-over. An Extension is not a place to tell the actor how to read the dialogue or to indicate some action happening on the screen.
There are a couple of standard Extensions: O.S. and V.O. "O.S." means "off screen" -- the character is physically present, but not visible on the screen (behind a door, under a bed, etc.). On TV shows, you'll sometimes see O.C. (for off camera) instead of O.S. "V.O." means "voice-over" -- the character isn't on the screen but we hear the voice (a narrator, a voice on an answering machine, etc.). This is called a voice-over, because the actor records her/his voice at a time other than when the scene is being shot, and the voice is placed "over" the scene. It's still a voice over, though, if the actor records her/his voice and the recording is played back as the scene is being shot.DR. JOSEPH (O.S.) Hey, someone let me out of this freezer!WALLY (V.O.) That was when I knew... plug in the freezer!
The Extension is uppercase and enclosed with ( )'s. There is one space between the end of a Character Name and the opening parenthesis of an extension.
Couldn't be easier. Once you've typed your Character Name, just press <Tab> or the "(" key.
Scriptware will pop up a list of Extensions. Just start typing (one letter will usually select the one you want off the list... if what you want isn't on the list, just type the whole thing, then press <Enter>... it'll be on the list the next time you need it).
Once you've typed your Extension, <Enter> will put the cursor in the right place for you to type Dialogue.