If you cast an adventurer card as an Adventure, use only its alternative characteristics to determine whether it's legal to cast that spell. For example, if you control Johann, Apprentice Sorcerer ("Once each turn, you may cast an instant or sorcery spell from the top of your library") and Promise of Aclazotz is on top of your library, you can cast Foul Rebirth, but not Promise of Aclazotz.