YUSSSSS.
I just had a breakthrough.
It isn't much, but I've been wondering about this for the past, what, 5 months?
So I'm really happy I finally figured it out.

I guess I'll explain what I found out :3
When making a flash game, you can move a movieclip with the < and > buttons.
Now with a circle as your main character that wouldn't be a problem, but once you use an actual character with a walking animation, you'll notice that the movieclip won't change directions if you walk left or right. So your main character would use the same animation for walking left and right.

The normal code for moving a character would be:

if(Key.isDown(Key.RIGHT)){
this._x += speed;
}
if(Key.isDown(Key.LEFT)){
this._x -= speed;
}

Now finally!
After a lot of stupid research and mistakes, I found out!

Next to the functions of x and y, you also have xscale and yscale.
I never knew D:

So yah, if you put

if(Key.isDown(Key.RIGHT)){
this._x += speed;
this._xscale = -100;
}
if(Key.isDown(Key.LEFT)){
this._x -= speed;
this._xscale = 100;
}

The character suddenly turns around when you press the other button.


Tiny discovery, but I am really happy.
And I also have a journal now that isn't completely random.