The most common technique is asset swapping. Instead of animating the mouth pathing manually, animators assign specific mouth assets to specific phonetic sounds (visemes): A large round open circle for vowels.

A flat, closed line representing pressed lips.

To do this:

The BFDI mouth asset is a testament to the power of minimalist design. What began as a quick solution for two teenage brothers trying to animate a YouTube web series has evolved into an iconic staple of internet animation history. It proves that you do not need Hollywood-level budgets or hyper-realistic graphics to tell engaging, hilarious, and emotionally resonant stories—sometimes, all you need is a gray circle, a couple of stick limbs, and a perfectly timed black-and-white smile.

The assets strictly use solid black for outlines and shapes, and solid white for teeth or tongue fills.