Boromir, Warden of the Tower

If a spell is cast with an alternative cost that includes mana (such as flashback), Boromir's second ability won't trigger and won't counter that spell. Similarly, if a spell is cast without paying its mana cost, but the player did pay mana for additional costs such as from kicker or from Thorn of Amethyst, Boromir's second ability won't trigger and won't counter that spell.

If the creature you choose as your Ring-bearer was already your Ring-bearer, that still counts as choosing that creature as your Ring-bearer for the purpose of abilities that trigger "whenever you choose a creature as your Ring-bearer" or abilities that care about which creature was chosen as your Ring-bearer.

Players may cast spells that they know Boromir, Warden of the Tower will counter. Any abilities that trigger when spells are cast will still trigger and resolve if appropriate, and any effects that count how many spells are cast will still count those spells if appropriate.