Why James Bond Was Wrong
Before you can find a cocktail menu, the bartender appears in front of you, drops a napkin on the bar top, and looks at you with eyebrows raised. You tell the bartender you want a cocktail, but you're not sure what kind. And if he or she is worth anything at all, they will respond with assistance instead of annoyance, guiding you Socratically toward the perfect drink for that moment, which inevitably begins with some form of the question: "shaken or stirred?”
There are plenty of ways you can mix a drink: you can build it in the glass, roll it or carbonate it, you can throw it over your head and catch it behind your back if you've got a mind to. But the two most common ways, by far, are shaking and stirring. So what's the difference? Why not just pour the ingredients on ice and be done with it, like a rum and coke?
Here's why: there is a secret ingredient in every cocktail you've ever enjoyed, and it's not salt or sugar or St. Germain, it's water. When we're talking about ice, there is no chilling without dilution, and vise versa. Ignore the bartender who tells you that his ice chills without diluting, because he's wrong. Big ice cubes dilute slower, but also chill slower: use them to keep a low temperature rather than attaining one.
Cocktails are meant to be diluted. It's implicitly built into the recipe. So when you stir or shake, you are melting the ice, lowering the temperature of your drink, and diluting it all at once. Additionally, when you shake, you're aerating it, thinning the texture and making it feel lighter. This is an enormous difference, and there's little to no wiggle room. If someone stirs my margarita, I know they don't know what they're doing. If they shake my Sazerac, I'm walking out.
And so, the difference:
Stir when all the ingredients in the cocktail are alcohol, i.e. no mixers, no juice, no cream, no eggs. Manhattans, Old Fashioned’s, etc. Stirring is gentle and maintains the silky viscosity inherent in such drinks, which is without a doubt one of the most enjoyable parts about them. It renders a cocktail with roughly 30% water.
Shake when there's juice, cream, eggs, or some kind of other mixer. Shaking aerates as well, thinning the texture and making it brighter and lighter, and makes the final drink roughly 40% water.