Generosity. Unyielding unflinching generosity. Giving the other NPCs and PCs the attention they need.

You can also look for ideas in books like Hillfolk (has ideas on how a “dramatic pole” can help, i.e. being torn between two conflicting values like home life vs work) or Play Unsafe (such as playing with status, being a noble or a servant). Play Unsafe also has the wonderful tip to not be afraid to be boring or obvious when improvising, to say the first thing that comes to mind as opposed to trying to force a creative or unique idea, because what you think is gonna be boring may well be super interesting to the other players (and when it's not, it's at least something basic and fundamental they can easily build on).

You can also collaborate with another player for a type of relationship with their character like being their body guard, religious follower, sibling, servant, spouse, teacher, or student. (Only if they’re into it, of course!) Some storytellers over the years have used extremely shallow and two-dimensional characters to great effect over the years simply by having a cast of characters collide with each other—people meeting other people is great story fuel.