One of the best pieces of writing advice I've ever read came from Phoebe Waller-Bridge, who said she always tries to give a character three things to juggle in any given scene.

For example: they’re trying to order a sandwich, the deli is unbearably hot, and they’re trying very hard not to let on that they plan to rob the bank down the street in twenty minutes.

The thing I love about it is how much of the character's voice falls out naturally as a result! I can almost hear them before even writing a line.