Have a scene of the girl spending time with her boyfriend, then a scene of her spending time with whoever she's cheating with.
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The most obvious way is to actually show the woman with the other man. But if you're going for more subtle, perhaps she receives a gift in the mail that is clearly romantic and that she is trying to hide from her boyfriend. Or she receives a text from the guy asking her to meet her somewhere, and she lies about the text to her boyfriend.
Show her returning at an odd time, possibly in the middle of the night, somewhat disheveled and trying not to be noticed.
Ahr river
When talking to the cheatee, have her not be entirely absorbed in conversation. Side glances. Yessing. Then if you have the person that she's cheating with, have them smile at each other. It's all it takes. Smile at each other for a few seconds too long.
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How can I show, visually and without dialogue, or without a lot of dialogue that a girl is cheating on her boyfriend?
Keep in mind this is a movie script, a visual medium.