One of the four pillars of accent work is People aka Cultural Context. This refers to the fact that a person’s accent and voice quality is heavily dependent on their cultural, social and socio-economic background. For example, an upper-class person doesn’t sound like a person from a lower socio-economic background. Likewise, a teenager’s accent is very different from a 50-year old person’s even if they are from the same town. This pillar of accent work is crucial for acting. To deliver an accurate performance you need to know your character’s cultural background because it is indirectly reflected in their speech.
In the above video you can see a great example of this: Margot Robbie as Tonya Harding in I, Tonya. In that movie, Robbie plays her character at different stages of her life and she perfectly reflects the age difference with her voice. The older a person is, the stiffer their vocal cords become, and therefore a person’s voice sounds deeper as they grow older. Margot Robbie nails that aspect in I, Tonya – besides also doing a spot-on Northwestern accent in general.
[FYI: The above video is a delight to watch in its entirety. Erik Singer offers wonderful analyses of soooo many movie accents! Check out all his videos if you’re looking for good (and not-so-good) examples of accents in movies.]