To answer this question, we start with the second part of the card’s text:
Errata and a clarification for Heedless Revelry were issued in CRF 13:Alternatively, may be revealed as an on-guard card on a company after the successful play of an item, ally, or faction. Tap all untapped non-Ringwraith, non-Wizard characters in the company.
*presumably this period should be outside of the quotation marksCard Erratum: Change «after the successful play» to «in response to the play[.]»*
Does not interfere with the playing of the card it is revealed in response to.
Therefore, the second paragraph of the card should actually read:
In practice, this means that Heedless Revelry can be revealed in response to a player declaring that a character in the company is being tapped (as an active condition) to play an item, ally, or faction. Furthermore, per this CRF rule:Alternatively, may be revealed as an on-guard card on a company in response to the play of an item, ally, or faction. Tap all untapped non-Ringwraith, non-Wizard characters in the company.
Heedless Revelry is treated like any normal on-guard card, i.e. declared and resolved immediately prior to the chain of effects during which it was revealed. This means that, when it is revealed, its second effect immediately taps all non-Wizard, non-Ringwraith characters in the company and the resource player doesn’t have the option to respond because Heedless Revelry has already been declared and resolved prior to the current chain of effects.A revealed on-guard card retroactively takes effect as though it were both declared and resolved immediately prior to the chain of effects during which it was revealed.
Most importantly (and only slightly paradoxically), the character who tapped to play the item/ally/faction is not affected by Heedless Revelry because the character was already tapped as an active condition of declaring the resource being played, and thus, that active condition is still met even after Heedless Revelry resolves and the resource being played can still resolve as normal, including any relevant influence checks and so forth. This follows the CRF clarification cited above:
As a final note, revealing Heedless Revelry does not prevent an additional minor item being played by a Wizard/Ringwraith since those characters would remain untapped, but all other characters would be tapped prior to the resolution of the initial resource and prior to when the additional minor item could then be played.Does not interfere with the playing of the card it is revealed in response to.