I'm up to DM soon, but I've thought up an issue I'm unsure how to tackle

The Face of the party being the only roleplayer because the DM doesn't acknowledge anyone else.

Former DM did this, seeing any (other) character development was superficial at best (mostly because people were thrown for a loop the few times it occurred that he didn't want to talk to the low-charisma no diplomacy skill-ranks character)

How do I avoid tunnel vision and best allow everyone to have their time in the sun?

Pic related, it's mfw pondering this situation