Unrolling and reading a scroll requires an Action.
However, how about this: you use your Action to read a scroll of Counterspell, and then you have Counterspell as a Readied Action.
If you use an Action to read your scroll of Unearthly Smite, and you then use your Reaction on an Attack of Opportunity, could the Smite apply to the AoO? (You might be relying on your bard ally, who sees you using the scroll and thus casts Dissonant Whispers.)
However, how about this: you use your Action to read a scroll of Counterspell, and then you have Counterspell as a Readied Action.
If you use an Action to read your scroll of Unearthly Smite, and you then use your Reaction on an Attack of Opportunity, could the Smite apply to the AoO? (You might be relying on your bard ally, who sees you using the scroll and thus casts Dissonant Whispers.)