That mystical 就 again Here is a sentence:


  厨房壁炉架上的那面镜子就把哈利吓了一跳


Below are my translation variants. Bold letters should be a 'jiu' english analog here.


The mirror on top of the kitchen's fireplace scared Harry the most.
Even a (simple) mirror on top of the kitchen's fireplace was enought to scare Harry.
The mirror on top of the kitchen's fireplace scared the hell otta Harry.


What does it mean with 就 and what about without it? I know it should emphasis something, but my russian brain just can't get hold of it.

One more example of the same construction: 

Context (paragraph before target):


  他父母留给他的一点可怜的财产，存在古灵阁的地下金库里。当然，他的钱只能在魔法世界里使用，你不能在麻瓜的商店里用加隆、西可和纳特买东西。哈利从没向德思礼一家提起他在古灵阁的存款，他认为他们虽然惧怕与魔法有关的一切，但这种恐惧大概不会扩展到一大堆金币上面。


THE GAP HERE!!! I mean J.K. started the target sentence from a NEW LINE!!!


  到了下星期三，韦斯莱夫人一大早就把他们叫醒了。他们每人匆匆吃了五六块咸肉三明治，然后穿好外套。韦斯莱夫人从厨房壁炉架上端起一只花盆，朝里面看着


so you see... it's given (in the book) WITHOUT any context as far as I understand the word context :)!!!
Relating to your first sentence: 韦斯莱家却充满了神奇和意外。厨房壁炉架上的那面镜子就把哈利吓了一跳。

This 就 is the 12th meaning of 就 as an adverb in 《现代汉语词典》:


  表示事实正是如此：那就是他的家 ㄧ这人就是他哥哥ㄧ幼儿园就在这个胡同里。


It's meaning  12e in 《现代汉语规范词典》


  表示事实如此。 这儿就是我的家 | 车站就前面。


It expresses that the facts or the situation are really like this.
It can be seen as an illustration of or a proof for the previous sentence.

Relating to your second sentence: 到了下星期三，韦斯莱夫人一大早就把他们叫醒了

This pattern of 就 is explained here: https://chinese.stackexchange.com/a/1906/540
I'm not going to copy the answer I gave there :-)