Emma Shih published a video showcasing the reference shots she created to animate her parts of Into The Spider-Verse. Shih acts out all the scenes herself in order to observe the exact body language of, say, a teenager realizing his mentor is going to get him killed.
Of course, animators haven't always had the luxury of recording themselves on their phones. Reference work has been used by artists to portray any human figure more complex than stick figures hunting mammoths (with a few disastrous exceptions). Traditionally, animators still do it in the same way classic painters did to create their superheroes (i.e. Greek gods): by observing hired models in various poses. Though back then, those models were often prostitutes, as they're a lot easier to convince that to accurately capture Athena, you had to take off your underwear.