One example from the film I will use to demonstrate my analysis of this performance is Regina's big monologue on Janis Ian where she sells Regina's malice in a short span of lines:
As Regina goes on her diatribe about Janis and the slow descent Janis went on, the way McAdams handles her dialogue makes it seem like Regina is unaware of the severity of Janis' slow descent. Because of how Regina jumped to conclusions about her ex-best friend being a lesbian and brushed off both their mothers fighting, it seems as if Regina is vicious yet blind sided by her own naivete and McAdams sells the character, as well as the monologue, perfectly.
But something about this next clip strikes me when I think about it:
When McAdams delivers the line "Sweatpants are all that fits me right now", she manages to speak it with layers of subtext. As Regina is rejected by her own clique, McAdams reveals her more human side, showing how hurt her character is. Even though Regina had less redeemable qualities, she still gets her feelings hurt like everyone else. It seems like we're not supposed to feel bad for her, but in this scene, we almost can't help it. But immediately after that "Sweatpants" line, McAdams delivers a funny quip with "Fine. You can walk home, bitches!", proving how she nails the character's comedic timing with a tint of pathos.
So I think the two scenes shown above reveal the rather hidden nuances to the character that McAdams nails: her comedic timing, her humanistic vulnerability, and her malicious naivete.
Those are my thoughts on the performance by Rachel McAdams. Whether you agree or disagree or would like to offer your own analysis of this performance, please feel free to write your thoughts in the comments section. Thanks for reading!