This is pretty well known, but in the original dr who and the daleks episode there is a point where daleks are talking over the videophone to a group of other daleks, but if you look closely only the daleks that reply are real, the rest are cardboard cutouts!
My favourite Doctor Who fact is that if you look closely at any Colin Baker episode, the shouty bloke with the curly hair and daft coloured costume is made of wood...