It is actually not possible to see any face you've never seen in real life in a dream, because anything that happens in your head is purely you. For
example, when someone says to themselves "You're being stupid today" Who are you talking to? You've just split yourself into two personalities.
The one that takes instruction and the one that tells you what you're doing wrong.
As for the dreams, sometimes when someone sees a picture and sees the figure in their dream, the figure can tell them things that only their deepest
subconscious has been trying to tell them, so they assume that they are being spoken to by someone else. And since it is unknown what this figure
actually looks like, it only proves that you're only seeing it because you want it to be there.
And to the guy that talked about flying in his dreams- that is called lucid dreaming. It's just where your body goes to sleep but your mind does not.
What the mind sees is so real, that this is where most beliefs of alien abduction come from. There is actually a way to induce it, though I have not
tried it because in one of the stages, if you accidentally open your eyes then your body is still still asleep, and seems paralyzed to you. However,
your mind is both still awake and in a hyper active dream state, so you are able to see things in the room with you. This would make anyone very
scared, since they are afraid they cannot move, and thus produces (while you're awake) images of disturbing things that look very real.
Unfortunately, when we can't explain something we like to think it's celestial. This has happened for thousands of years. But the truth is that we
don't even fully understand our minds yet, and places of it can be accessed in dream state that we didn't even know existed.
Hope this helped