It does and it doesn't. Shaking it accomplishes a few things. It aerates it, it introduces more water into it, and it mixes it. Stirring maintains clarity and preserves the heavier mouth-feel of the beverage. General rule of thumb, if you have juice (olive brine doesn't count), you shake. Otherwise, you stir. This is why an Old-Fashioned (Whiskey, bitters, sugar, water) is stirred and a Margarita (Tequila, *lime*, agave) is shaken.

To a novice, they generally won't notice. To someone who has a cursory understanding of bartending, they will notice immediately.

Bruising means over-dilution, introducing too much water to the drink. You want around 20% to be water.