(None of this advice really applies to dialogue. If it’s in your character’s voice, they can use whatever words they like!)
Suddenly
This one usually makes people’s list for things to cut. “Suddenly, the door opened.” Turns into, “The door slammed open.”
As always, we want to make the readers think wow that was sudden! Instead of just telling them so.
Saw/Heard + Felt
I already explained this in my post here!
Seemed
There’s a use for seemed in writing when your character is surprised, assuming, or guessing at something. “It seemed impossible.” “The noise seemed to travel for miles.” Etc.
However, when guessing at someone’s emotions—or a group of people—it’s better to just describe what those people look like. So “He seemed happy” turns into, “he grinned, bouncing on his feet.”
Really/Very
Instead of “The really big house” try, “the huge house.” Or “His hair was very dark.” Turns into “His hair was inky black.”
That
If you can take ‘that’ out of a sentence, it usually is stronger than if you don’t. “It was the best cake that she’d ever had!” turns into “It was the best cake she’d ever had!” It reads a bit less clunky.
Then
Then can be used sometimes, but it’s one of those words that’s easy to overuse. To cut out a lot of your ‘thens’ you can replace them with “and” such as, “He left the house, then got into the car.” Turns into, “He left the house and got into the car.”
Down/Up
“He sat down” is redundant. “He sat” means the same thing. Same with “She stood up.”
I chose the ones I find the most important, but there’s tons of other words that can be unnecessary or bog down your prose. Let me know which ones I missed! Good luck!