This is a topic that always waivers depending on your opinion, there never really is a right answer. But the main thing isn't whether or not you follow the movie, show, or series you're writing about, it's the quality.

I, personally, don't care either way. My stories are a mix between original storylines and storylines that follow scripts. It all depends on the ideas you have. Let me go through some pros and cons for each.

Original Stories

Pros

- Show off your creativity

- A new perspective on the character

- Have more creative liberty 

- Are more original

Cons

- They're harder to mantain

- They're easier to make a character go off their normal personality

- They can be confusing

- Plot-holes are more common

Scripts

Pros

- Plot-holes are less common

- Easily understandable

- Easier to write for

- Very hard to change a characters personality completely

Cons

- Not always scripts out there

- The inserted character can seem pointless

- No changes from the movie whatsoever (exact copy)

- Has the same plot-holes the movie does

I have something to stress for both

Following scripts: The inserted character has to be there for a reason. They can't have near to none lines and only exist as a romantic interest. Give them a storyline, depth, character growth. Make it seem like they really could be a part of that universe. Another thing, don't replace another character with yours. That's pointless. Sure, you can trade dialogue at some points, but taking out, say, Peggy Carter, and putting in your character as her with the same lines, and personality with the only difference being name and looks, is a big no. 

Originals: Have a storyline ready. I can't stress this enough. In fact, I'm going to make a whole chapter about this. 

Hope you enjoyed! Feel free to request and ask questions! Love you all! Byeee!

~ Lydia