Lots of advice for playtesting, and its importance to the game design process (video):

https://www.youtube.com/watch?v=qBezywgh2yM

Game tempo: How the kind of progress that players make during a game affects the way the game feels (video)

The different kinds of playtesters, and how to interpret their feedback (video)

“You’ve made a game when people have played it.

You’ve made a GOOD game when people want to play it again.

You’ve made a GREAT game when people tell stories about playing it to their friends.”
— Omari Akil

Tips for running virtual playtests (video)

Advice for co-designing games, with an emphasis on the social and emotional aspects of it (video):

https://www.youtube.com/watch?v=9q5gH7of8I8