Filter words are those pesky things that can drive a wedge between the reader and the POV character, creating distance and lessening the impact of the scene. If you've been following my blog, you will know I hit a wall half way through my WIP where I felt as though I "lost" my main characters unique voice.
I've put it down to a couple of things, one: hitting writers block and taking a week off from writing, when I came back I felt as though I'd lost that connection with how my MC would naturally act and talk (to a degree). Two: wondering whether I should have written the whole novel in first person instead of third person, which leads me to Three: in third person I felt like I was struggling with the distance. I asked myself why, and one of the reasons I gleaned was that I was creating too much distance for the reader, and as a result it felt as though I was reporting my main characters actions, and noting her feelings instead of allowing myself and the reader to experience those things alongside Grey (my MC).
So what on earth are filter words? They are words that report what the character does and note what the character feels, and are typically the following: saw, watched, heard, felt, noticed, realized, thought, wondered, looked, decided. Even touched, tasted and smelt can distance the reader.
When you are writing in third person deep, like first person POV, you do not need to distance the reader; you are always in the main characters head for that particular scene or chapter. So, for example, when you are writing internalization, you don't need to say "she thought" at the end of the sentence, because the thought can not belong to any other character. Filter words are not only unnecessary (in most cases), but they also dull the impact of the scene by putting more distance between the reader and what that character is doing and feeling.
Lets look at a couple of examples of sentences with filter words, and then compare it to sentences where the filter's have been taken out:
"Keta flipped the page of her book. Yet another late night, she thought, cringing at how dark the bags under her eyes would be in the morning. Just one last chapter. She heard a creak break through the silence; she felt every muscle freeze as she heard the windowpane shake and groan at the effort of being pulled open after so many years. He had come back she thought."
Without the filters:
"Keta flipped the page of her book. Yet another late night, she cringed at the thought of how dark the bags under her eyes would be come morning. Just one last chapter. A creak broke through the silence; every muscle froze as the windowpane shook and groaned at the effort of being pulled open after so many years. He had come back."
Filter words can make a sentence clunky, it reminds the reader - hey, you there, yes you with the book in your hand, you are reading this! Instead of allowing them to actually experience the events alongside the MC, or allowing them to glean from their reactions how the character feels instead of being told she felt something.
That's not to say that you should necessarily go through your manuscript and cut them all out entirely, there may be some circumstances where you're actually trying to create distance or where you do need to emphasize something by reminding the reader that the MC saw something, heard something, or felt something. But if you are doing it with every single sentence as in the first example above, it can get annoying for the reader, and make them not care about your characters because of the distance created. Which you definitely do NOT want. In my first draft I let them sneak in, but cut those pesky filters out during editing.