The job of a good scriptwriter is to make the dialog flow straight from the page to the screen.
In a high budget production, this is what happens 99% of the time.
Any changes an actor wishes to make will be discussed with the director. Any changes made will be noted by the script supervisor, so that when it goes to edit, there is an awareness of where any digression from the original is.
In an amateur or student production, people tend to wing it a lot more. The script hasn't been through the same kind of editing process beforehand & people are more free to make things up on the spot. There may not even be a script supervisor.
This assumes the entire production isn't already designed to be highly improv. I've worked improv movies where there's no script supervisor, because there's no scripted dialog at all.
Your job as an actor is to take what they've written and bring it to life. Their job as a scriptwriter is to give you dialog that works. Without seeing the script or hearing the performance, that's all anyone can say. If this is part of a student project, then take it to your acting teacher for guidance.
There's a joke [tough to get this to work in text, you need the emphasis in speech really…]
"Act like you've never acted before! ..ermm… no, not like you've never acted before…"
It's not the actor's job to take "Is this a dagger I see before me?" & turn it into "Right, I've got me penknife out… now what?"
Shakespeare, to modern ears, feels very stilted, yet every night, probably hundreds of actors around the world make sense of it to anticipatory audiences.