What does 'underwritten" mean in the following sentence?

I am asking about the meaning of 'underwritten' in the following sentence? The film received generally positive reviews from critics, with praise for its visual effects and action sequences, but with criticism towards its underwritten human characters.

Thanks in advance.

Solution 1:

I might prefer to see 'under-written' there, to avoid confusion with 'underwritten' as used in the insurance business. It's the kind of word familiar to movie buffs and people who read reviews in e.g. Variety:

"With multiple authors but no clear voice, the clumsy “Mulan” script often puts plot above character, depriving Mulan of a robust personality. Defined by her determination, she mostly keeps to herself, which deprives her of meaningful human relationships during the mid-section of the film. Underwritten as she is, Mulan is handily upstaged by Xianniang (Gong Li), a powerful sorceress invented for the film who gives Mulan a strong female adversary. "

Mulan (Variety review)