Especially once we become more aware of the ways that bias can manifest, we often notice it more in other people's behavior. It's important to know how to be a good ally in these situations to avoid unintentionally doing further harm.
The person harmed might be you, or it might be a student, friend, etc. The first priority is to prevent further harm and figure out what the person harmed needs. This will vary depending on the people involved and the circumstances. Especially if this happens in a public space where you don't know everyone around you, be mindful of safety. Sometimes a more direct approach can lead to retaliation.
Allyship can take many different forms. If you are standing up for someone else, here are a few forms that might take:
Offering an alternative point of view to push back on the bias voiced
Indicating separately to the person harmed that you noticed what happened and are there for whatever form of support they need
Following up with the person who expressed the bias to make sure they understand how harmful their behavior is (this is most effective when you share their privileged identities)
Especially if you notice something that is part of a pattern of behavior, try asking the person or people harmed how they would like you to handle it when it comes up again
Remember the Platinum Rule. Not everyone wants to be publicly defended, and no one needs you to "rescue" them. The point is to support them, not to prove to everyone that you are on the right side.
If you witness bias against someone else, think about the bigger goal and your role in the situation. The first priority is to take care of the person harmed. The second priority is to do your best to prevent further harm. In some situations the best you can do is to create a buffer or an exit, providing the person harmed with options to avoid contact with the person causing the harm. However, the ideal outcome is for the person who caused the harm to reflect on their behavior and learn to do better.
When thinking about educating others on their own biases, it's helpful to think about the difference between "Calling Out" and "Calling In".
Calling Out is a more direct naming of what someone did and how it was harmful. It usually feels more confrontational, and is sometimes the more effective option when someone is dominating a space with aggressive or bullying behavior.
Calling In is meant to prompt reflection, and is often more subtle. It can sometimes take the form of a question, and in many cases allows the person to save face. It is often the more effective option when you think there is hope that the person was unaware they were causing harm and is potentially open to learning.
Here are a few examples:
A White student describes a Black student as "too aggressive": Calling Out could sound like, "That's a really problematic stereotype." Calling In could sound like, "I haven't noticed that. What makes you describe them as aggressive?"
A cis-gendered student misgenders a TA who is a transgender woman: Calling Out could sound like, "Her pronouns are she and her." Calling In could simply be making a point of using the correct pronouns for her.
Especially when taking more of a Calling In approach, drawing on your understanding of Emotional Intelligence and Effective Feedback can be particularly helpful!