I can assert myself and stand up for myself and others.

Assertion in practice is one flat sentence at normal volume. She suggested that ten minutes ago. I can't take that on this week. That isn't what happened.

It gets rehearsed as a confrontation and almost never becomes one. The difficulty isn't force; it's the two seconds afterwards, when the room hasn't reacted yet and you don't know whether it will.

Standing up for someone else is usually easier than standing up for yourself, which is worth exploiting. Whatever voice you'd find for a colleague being talked over is a voice you already own. It works pointed the other way.