Not giving readers enough detail, emotion, or development - leaving them unable to fully see, feel, or understand your story.
Underwriting is the problem of providing too little on the page: not enough sensory detail for readers to picture a scene, not enough interiority to understand a character's emotions, or not enough development for a plot point to land with full impact. Where overwriting smothers the reader, underwriting starves them. It often shows up as rushed scenes, flat emotional beats, and settings that feel like empty rooms.
Readers can't fill in what they don't have clues for. If you skip the sensory details of a setting, readers have nothing to visualize. If you rush through an emotional turning point, the payoff falls flat. Underwriting forces readers to do too much work, and most of them will simply disengage rather than imagine the missing pieces for you.
Writers who've been told their prose is too flowery sometimes strip out everything, including the details that matter. Cutting should be targeted, not scorched earth. Keep the telling details; cut the decorative ones.
Hemingway's prose is spare, but it's not underwritten. Every sentence carries weight and implication. Minimalism is choosing fewer words carefully; underwriting is just not giving the reader enough to work with.
The scenes that matter most to your story deserve the most space on the page. If your character learns their best friend betrayed them and you cover it in two sentences, you've underwritten the moment your reader was waiting for.
You can see the scene perfectly in your head, so it feels complete. But your reader doesn't have your mental movie. Beta readers who say "I couldn't picture this" or "this felt rushed" are flagging underwriting.
Find a scene in your draft where a beta reader (or your own instinct) flagged the pacing as too fast or the emotions as flat. Rewrite that scene at double its current length. Add sensory details, internal reactions, and small physical actions. Then compare the two versions and find the length that feels right - usually somewhere in between.