Conversely, here's Rufus Sewell, as Lord Melbourne, improperly dismounting his horse:
What if the horse reared up its head and your spurs hit its neck? The horse might bolt causing injury to mount and rider alike.
Here's a wiki and youtube instructions for all to learn:
Obviously, both John Wayne and James Caan learned how to dismount horses properly in "El Dorado"
Richard Cumberbatch also properly dismounts, hunchback and all, in Richard III