Guard dog tendency?

Skippy (9.5 months old) is just like any cavalier in that he loves meeting and greeting any and all strangers. We live in a multi-storey apartment building, and lately have noticed that whenever he hears a neighbor (or anyone in general) walking down the corridor outside our main door, he'll start low-growling and will let out a couple of angry-sounding barks. Is this normal Cav behavior?

It is normal dog behaviour.

Even little Kaley does that when she hears the postie or sees a passer by from her vantage point along the back of the settee and she's only been here a week - bless her!

That is definitely normal dog behaviour. We have two cavs who are as different as night and day. Our girl Belle is as relaxed and friendly as they come. When the door bell rings she just sits up from her usual napping position and smooths back her hair and puts on a big smile... as for our little boy Bob, he start barking and pacing telling everyone around him: "Someone's here! Quick! Be on your guard!!" It's pretty funny to see, we just shush him and tell him to do something like sit or lie down or go to his bed or we throw him a bone to distract him.

We were watching the series Band of Brothers recently and Bob was quite disturbed by the fighting scenes, he would jump off the couch anytime there was a fighting scene with bomb/gunshot noises. He would just growl and bark at those Nazis like they were in the room with us, it's really funny.