If I cast Mystic Reversal on Heedless Resurrection, do I get to bring something out depending on what my opponent sacrifices, or do I have to sacrifice something to bring something out from the graveyard?

When you cast Mystic Reversal on an opponent’s Heedless Resurrection, you gain control of that spell. As the new controller, you are entitled to make new choices for the spell, such as selecting a new target unit from your own trash to be resurrected.

The Ruling

Because the additional cost (sacrificing a friendly unit) is paid at the time of the spell's initial casting, you do not need to sacrifice a unit when you take control of it. You gain the benefit of the spell’s effect using the cost already paid by your opponent.

Why this is the case:

1. Gaining Control and Making Choices: Per the card text for Mystic Reversal, "Gain control of a spell. You may make new choices for it." When you take control of the spell, you become its new controller, allowing you to choose a new target unit from your own trash to be resurrected.
2. Costs are Paid upon Casting: The requirement to "kill a friendly unit" is an additional cost to play Heedless Resurrection. This cost is paid by the original caster when the spell is placed on the chain. By the time you use Mystic Reversal, the cost has already been satisfied.
3. Spell Resolution: Once you gain control, the spell resolves according to its text using your new choices. Because the cost was already paid during the casting process, you do not need to pay it again to resolve the effect.

Sequence of Events:

1. Opponent plays Heedless Resurrection. They pay the cost (killing their unit) and declare a target from their trash.
2. You react by playing Mystic Reversal targeting their Heedless Resurrection.
3. Chain Resolution: Mystic Reversal resolves first. You gain control of Heedless Resurrection.
4. New Choices: As the new controller, you choose a target unit from your own trash.
5. Heedless Resurrection Resolution: The spell resolves, and you resurrect the target you chose from your trash. No further cost payment is required.

Note: I could not fully verify this ruling against the retrieved rules.