When your client’s characters are one-dimensional, remember that we can impose perspective, motive, and agenda to ameliorate the plot holes and implausibilities they create. And starting with agenda, bear in mind it’s typically one of three varieties: maintain the status quo, accept evolution as it naturally occurs, or actively effect change.
In a coming of age, Romance, or even thriller, for example, the Antagonist’s agenda is often from the maintain grouping, whether they’re radically fundamentalist, staidly traditional, or simply conservative. That spectrum accounts for about eighty percent of all human beings, so it makes for a wonderful agenda base.
You’ll likely only use an agenda from the second category, which constitutes about twenty percent of the population, for secondary characters, to be honest. But ten percent of category two folks fall into its subset; “I want change and I want it now!” You can probably pull most Protagonist agendas from that “effect” subset, thereby creating natural conflict/tension with anyone of the maintain variety, and a spectrum of impatience (or humor) from or with anyone of the accepting sort.