If I didn't believe in you
We wouldn't be having this fight
If I didn't believe in you
I'd walk out the door and say
"Cathy, you're right"
But I never could let that go
Knowing the things about you I know
Things, when I met you four years ago, I knew
It never took much convincing
To make me believe in you
rachel finds jamie wellerstein to be such an interesting character in musical theater. he is so easily painted the villain because of, spoiler alert: being a terrible husband, but a closer read of the show reveals that both jamie and cathy have their shortcomings that ultimately lead to the end of their marriage. she finds their dynamic interesting to dissect, and cathy is one of her dream roles, but she secretly finds jamie to be a much more juicy role, and his songs are really fun to sing, too.
so rachel chooses if i didn't believe in you and puts her whole jam-ussy into it, acts the hell out of it, and of course, is dressed a little bit soft-butch, just for the fun of it, because no casting director is ever going to let her be jamie.
click here to see a video of a woman covering the song to perhaps get an idea for what this might look/sound like.