I'm going to lie about watching a movie so my significant other would enjoy it more
Why would you do something like that?
I'll say I know nothing about it so she can explain it to me during the time we're watching it
You should just be honest with her. She is not going to trust you if she finds out.
I don't want to spoil it for her, thats why I'm pretending that I don't know about it.
Ah, okay, I got you. I hope you both enjoy the movie.