in Editing
Filter words: saw, heard, felt, noticed
"She saw the door open" puts a character between the reader and the door. Often, the sentence is stronger without her.
"Filter words" is an editing term for verbs of perception and thought that place a character between the reader and what is happening. Saw, heard, felt, noticed, realised, wondered, watched, seemed, thought, knew. They are not errors, and they cannot all be removed. But they are often one of the easiest ways to make prose feel more immediate.
What filtering does
Compare two sentences, both in close third person with a character called Ben.
Ben heard a car pull into the drive.
A car pulled into the drive.
In the first, we are told that Ben perceived something. In the second, we perceive it with him. Because the story is already in Ben's point of view, readers assume anything described is something he notices. The "heard" adds a layer of distance without adding information.
The same applies to feeling. "She felt the cold seep through her coat" versus "The cold seeped through her coat." The second is shorter and puts the reader closer to the sensation.
Why writers use them
Filter words appear for understandable reasons. In drafting, they help the writer stay anchored in a character's head. They also feel careful, as if the prose is being precise about who knows what. And in speech we narrate our perceptions all the time: "I saw him come in", "I felt awful".
In revision, many of these anchors can come down, because the point of view is already established. The reader does not need reminding, sentence by sentence, whose eyes they are looking through.
When to keep them
Sometimes the act of perceiving is the point. "She noticed he had taken off his ring" is doing something that "He had taken off his ring" does not: it tells us she was paying attention, or that she saw it before he meant her to. The noticing is an event.
Filter words also help when a character is uncertain. "She thought she heard footsteps" says something about her doubt that "Footsteps sounded on the stairs" does not. And they can be useful when shifting attention within a scene, marking a change in what the character is focused on.
In first person, filter words can also carry voice. A narrator who constantly says "I noticed" may be characterised by that habit. The question, as ever, is whether it is chosen.
How to revise
Search for the common filter verbs and look at each instance. Ask: if I remove the character's perception, does the sentence still make sense in context? Does anything important get lost? If not, rewrite to describe the thing directly.
Watch for "could": "She could see the river", "He could hear shouting". This construction often doubles the filtering and can nearly always be simplified to "The river…" or "Shouting…".
Also check "seemed" and "appeared", which filter through uncertainty. Sometimes the uncertainty is right. Often it is a hedge that the writer does not need.
Done with care, this is one of the more satisfying revisions, because the gains are visible sentence by sentence. The scene moves closer without anything being added.