For some cards, surge represents only an alternative cost, a discount that applies if you or a teammate has cast another spell this turn. Other cards, like Reckless Bushwhacker, have additional abilities or effects if you paid the surge cost to cast the spell.

Casting a spell for its surge cost doesn't change its mana cost or its mana value.

The other spell that you or a teammate cast can be one that's resolved, one that was countered, or (for instants with surge) one that's still on the stack.

The enchanted creature can still be untapped by other spells and abilities.

Containment Membrane can target and be attached to an untapped creature. It just won't have any effect until that creature becomes tapped.

If an instant or sorcery spell cast for its surge cost is copied, the copy is also considered to have had its surge cost paid.