It's hard to answer this question, because I have been all of the above at some time. But the one I have been the most is probably the bystander. I have witnessed bullying many times, and sometimes I try to stop it, sometimes I do nothing. Sometimes it is a friend and sometimes a stranger. Either way, this situation has happened alot to me.

Once at school, one of my friends was telling another girl that she was "too skinny" and saying she needed to "eat more." This exact thing happened to me once, and I knew how the bullied girl felt. I said "Maybe she's naturally skinny, and can't help it." Then my friend stopped teasing the other girl. I was glad I said that. I realized that alot of bullying is just because of misunderstandings. The bully didn't understand that me and the other girl couldn't help being how we were.

Another time, a boy in my class was being teased because his parents wouldn't let him go do something with his friends. I overheard the conversation and I wanted to tell the bullies to stop. But I didn't say anything. I still don't know why. I think it was because I didn't know what to say. But I regret not saying anything and wish I could have helped the bullied kids.
In conclusion, there are many different ways you could be a bystander. you could join in the teasing, stand up for the bully kid, or just watch. I think almost everyone has been all of these at least once.