Shaker, spoon, or just stir: basic cocktail-making techniques
Quick answer: there are four basic cocktail techniques — build (pour and stir), shake (shake with ice), stir (mix with a bar spoon over ice) and layer (pour by density). Which one you use depends on the ingredients, not habit.
Build is for cocktails with carbonated components (tonic, soda, cola) — shaking would knock the fizz out. Just fill the glass with ice and pour the ingredients in order.
Shake is needed whenever the mix includes juice, syrup, egg white or cream — ingredients of different densities that need to be emulsified. Shake for 10–15 seconds in a tightly sealed shaker or jar with a lid.
Stir is for spirit-only cocktails (Negroni, Martini): stirring with a spoon chills the drink without over-diluting it or adding air bubbles that would change the texture. Layer means pouring ingredients in order of decreasing density (syrup → liqueur → spirit) over the back of a spoon, to get a layered shot or cocktail without mixing.