Every filter word does the same job. It puts the narrator between the reader and the moment. "She noticed the door was open" carries the same information as "the door was open," but the first version adds an invisible camera operator nobody asked for. Cut the filter, and the reader is standing in the room instead of watching someone else stand in it.
What counts as a filter word
Filter words are verbs of perception and cognition: see, hear, feel, notice, realize, wonder, decide, remember. Writers reach for them to report a character's experience instead of rendering the experience directly. They aren't wrong on their own. A first-person narrator does sometimes need to say "I heard." The problem is density. When every sentence routes through one, the prose reads like a transcript of someone describing a movie instead of the movie itself.
Nine to watch for
- Noticed. "She noticed the light was fading" becomes "The light was fading."
- Realized. "He realized he was alone" becomes "He was alone."
- Wondered if. "She wondered if the door was locked" becomes "Was the door locked?" Or better, show her trying it.
- Felt. "He felt angry" becomes the anger itself: a clenched jaw, a door slammed too hard.
- Saw, watched. "She watched him walk away" becomes "He walked away."
- Heard. "He heard a crash" becomes "A crash." Let the fragment carry the suddenness.
- Decided. "She decided to leave" becomes "She left."
- Remembered. "He remembered the promise" becomes the memory itself, woven in as a fragment rather than announced.
- Seemed, appeared. The hedge words. "He seemed nervous" asks the reader to do the interpreting a specific detail should be doing: a shaking hand, a voice that cracks.
When to keep them
Cut on instinct and you'll sometimes remove information the reader needs. A first-person narrator's uncertainty, "I thought I heard something, but I couldn't be sure," is a filter word doing real work, because the uncertainty is the entire point. Density is the actual problem. Not the words themselves. Notice when one is standing in for a scene you haven't written yet, and write the scene instead.
A before-and-after
Pasted in, unflagged
He noticed that the room was very dark. He wondered if the door was locked. Everything was a deafening silence.
Rewritten
The room was black. He tried the door. Locked.
Same information, a third of the words. The reader is in the room instead of being told about it.
How to catch these in your own draft
Reading for filter words line by line is slow, and your brain skips past your own habitual words every time. The fastest method is a find-and-highlight pass. Search your manuscript for "noticed," "realized," "wondered," "felt," "saw," "heard," "decided," "remembered," and "seemed," one at a time, and look hard at every hit. Kaiyo Manuscript does this automatically. Paste a scene in and it flags all nine categories at once, along with passive voice and clichés, so you can see the pattern instead of hunting for it sentence by sentence.
Common questions
Should I remove every filter word?
First-person narrators especially need some. The goal is catching the ones covering for an under-written scene, not hitting zero.
Do filter words matter in dialogue?
Less. Dialogue tags and internal reaction inside dialogue-heavy scenes read differently. The rule applies hardest to narrative description, not to how characters talk.