Horse people of Lemmy- how easily can someone steal a random horse? (several questions)

I'm not experienced with horses. I know many are trained to work for just about anyone. They were used in wars and police still use horses this day and age for crowd control, so I guess they can stomach a bit of violence and chaos. But most domestic animals behave differently with people they're familiar with vs random strangers, so I presume horses would follow suit?

My question is inspired by countless movie/videogame scenarios in which there are a bunch of random horses tied, and a character just picks any, hops on and rides away.

Or, there's a fight, horse owner gets killed and the thief rides away on the horse.

Regarding horse behaviour only, are these realistic scenarios?

How likely is that a horse would resist being mounted by a random?

Wouldn't they be scared or angry if their previous rider just got killed right there?

Is it possible for an experienced person to tell at a glance how obedient a horse is? (How?)

These are my sudden horse questions. TIA