Details

It's all about the minor details. In the case of faking a bathroom trip, pay attention to the small aspects. Just flushing the toilet and turning on the sink is way too basic. Try to imitate the wiping noise. Make sure to keep a short delay between flushing and turning on the faucet. Make sure the water actually hits something or it will sound completely different.

These same principles can be applied when writing. It's all about the small details that will result in magnificent world building.