Our Gemara quotes a baraita that discusses mi’un – refusal – and how it works:
Our Rabbis taught: What is regarded as mi’un? — If she said, ‘I do not want So-and-so as my husband,’ or ‘I do not want the engagement that my mother or my brothers have arranged for me.’ Rabbi Yehuda said even more than this: Even if while sitting in the apiryon (bridal litter) and being carried from her father’s house to the home of her husband, she said, ‘I do not want So-and-so as my husband,’ her statement is regarded as a declaration of refusal. Rabbi Yehuda said more than this: Even if, while the wedding guests were reclining in her husband’s house and she was standing and waiting upon them, she said to them, ‘I do not want my husband So-and-so,’ her statement is regarded as a declaration of refusal. Rabbi Yosei bar Yehuda said more than this: Even if, while her husband sent her to a shopkeeper to bring him something for himself, she said, ‘I do not want So-and-so as my husband,’ you can have no mi’un greater than this one.
The Meiri examines each of the cases in an attempt to show that each one adds a new element that we would not have been able to understand from the previous one; in each case her statement belies her actions, yet we accept her statement to be mi’un. If she is being carried in a bridal litter to her husband’s house, circumstances seem to show that she is willing to accept the marriage. This would certainly seem to be the case if she is serving the guests, or if she is doing her husband’s bidding by shopping for him. Nevertheless, if she says that she is not interested in the marriage it is accepted as mi’un and the relationship is dissolved.
Another approach is to view these statements as teaching that mi’un need not be done in a formal court setting, and even a conversation with family members, guests in the home, and even a statement made to a shopkeeper will be accepted as mi’un.