Conflict generally comes in two flavors: Internal and External. They're easily defined. External conflict is guys with guns chasing you and trying to kill you. Internal conflict is your belief that you're trash and not worth saving - think of it as the psychological and emotional baggage that comes prepackaged with a character.
You already know I'm going to say that ANY conflict arises from character. Here's what I mean by that. In order to come up with plausible, interesting external conflict, I need to know exactly what kind of emotional and psychological issues my characters are carrying around. Why? Simple. External conflict exists to reflect and exacerbate internal conflict. Whatever you believe you cannot do, I, as author, must challenge you to do - and I have to make it terrible.
Example: you don't believe you can face a room full of zombie babies because you lost your baby to crazed, three-toed sloths six months ago. Guess where I'm going to drop you. I might also light that room full of rotting infants on fire with you in it - while you listen to all those raspy baby zombie voices whimpering in the gathering smoke. This is probably going to trigger you over the lose of your child and you'll have to make a horrifying decision. Kill all those creepy, crawly zombie ankle biters yourself to spare them the flames? Or run and let them all go up in the fire? You'll have to listen to their cries if you pick that last one. Or is that *your* baby's cry you hear?
External conflict should pick the scabs off the internal conflict sores. It should make characters bleed and leave them (at the black moment of the story) lying in a puddle of comingled blood and tears. Even if those are figurative.
By knowing what self-doubts, insecurities, fears and faulty beliefs linger within my characters, I know what kinds of external conflicts they have to face - anything that exposes those self-doubts, insecurities, fears and beliefs. Bonus points in plotting if the external conflict pushes a character past what she believes herself capable of withstanding. Such hard, implacable, external forces are necessary in fiction because novels compress the time it takes most mere mortals to change. Instinct tells us that without some pretty solid motivation, the hero is not going to just up and admit the error of his ways, resolve to be a better person and wander off into the sunset, no fuss, no muss. We have to rub his nose in his wrongheadedness. External conflict does that.
When I was planning and writing my first book, I knew the heroine had PTSD from having been a prisoner of war. Initially, it was a device that gave the hero some inroads into her psyche. In no way had I intended to hand that heroine back to the creature that had kept her prisoner. Yet. When I got blocked three quarters of the way through the book, it became clear that had to happen. It was her greatest fear and the greatest test that proved whether she'd learned her lessons from every other external conflict that had come before. (I hadn't wanted to give her back to the alien because in no way did I want the story to suggest that kind of thing could cure a complex issue like PTSD.) But hopefully, it's a reasonable example for how a character's own internal issues point up interesting external conflicts that will heighten those internal conflicts and make everything much, much worse.