Why does AI-generated text sometimes feel so stiff and robotic, even when I ask it to sound casual?

AI writing often sounds stiff because it's trained to predict the most statistically 'safe' word next, which leads to overly formal, generic phrasing rather than the messy, varied rhythm of human speech. When you ask it to be casual, it doesn't actually understand 'casual' the way we do. It just picks words from its training data that are statistically associated with informal requests—like throwing in a 'hey!' but then still structuring the sentence like a corporate memo. I've found the real problem is that AI defaults to what I call 'textbook voice': complete sentences, perfect grammar, no fragments. Real human conversation is broken. It's full of short bursts. Pauses. We start sentences and then we just... stop. To fix this, don't just say 'write casually.' Instead, give the AI a specific voice to mimic. For example, instead of 'write a casual email,' try 'write this email like you're texting a friend you've known for ten years—use sentence fragments, one-word answers are fine, and don't be afraid to sound a little unpolished.' A useful trick I use is to paste in a sample of my own writing and say 'match this tone.' It works far better than any adjective. For a deeper dive, see our guide on [why AI writing sounds too formal](/blog/ai-writing-sounds-too-formal). **Related**: How can I make ChatGPT sound more like me? | What's the best prompt to make AI writing less formal?