From the show's transcript (with thanks to NPR!):
DAVIES (Fresh Air contributor Dave Davies):Is it harder to get a script where you've got a small part and figure out the character? TOBOLOWSKY: Oh, it's one of the real challenges. It's what people don't expect as a difficulty of being a character actor. When you go back to the roles that have two names - Harrison Ford, Johnny Depp, those guys - everything they do is in the script, their entire day. You see them drinking coffee, you see them taking a shower sometimes. Their entire throughline is in the script. If you play their buddy, or worse the plumber of their buddy, you have a day, too, and you have a throughline to your day, too, but you have to do that work outside of the script. If - I know a lot of actors listen to this show, and I want to bring this up. One of the quick ways I use as a character actor to get into any part is I ask two questions: What is my greatest hope? What is my greatest fear? And usually you don't have a lot of time as an actor to study your part, but if you can answer those two questions, they will form a tightrope upon which almost any other question in the script can be answered, and those are the two I go for. Yeah, that is the work of being a character actor, doing that work off-camera.
I've added italics to show the spot that particularly struck me. Have you asked these two important questions of every character in your story? If not, give it a try and bring your manuscript's secondary characters to life!