Her face! Whether she is "angry", "sad" or "confused" the only part of her face that moves is her mouth. This isn't the Nicola McClean problem (botox), it is because what she is saying doesn't reflect what she is thinking.
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She fancied someone who reciprocated but actually really fancied someone else. People never seem to remember that Casey always knew Lee fancied Jasmine as she used to tease him about it and in the bolt hole despite seeing Lee literally salivate whilst watching Jasmine on the screen she still snogged him 

Criticising and disliking Casey doesn't mean siding with Lee. She is actually independent of him, supposedly.