The former American Idol seventh-season finalists will perform a duet during the eighth season's first results show telecast on Wednesday night, Johns toldEntertainment Weekly.
Johns and Smithson -- who finished in eighth and sixth place respectively on Idol's seventh season last year -- had originally been expected to appear on-screen during Idol's broadcasts of the season's Hollywood Week round in which they had served as "Idol Boot Camp" mentors.
However, their mentor footage ended up not making it into the broadcasts, causing AmericanIdol executive producer Ken Warwick to offer the pair the duet alternative.
"Ken [Warwick] felt bad about it," Johns told EW.
Johns said that he and Smithson had received their invites to perform on Sunday and added that they had not yet decided on which song they would perform.
As for his return to the Idol stage, John admitted to being excited, but also having some nerves about seeing the show's judges once again. However, the upside to maintaining a relationship with the Fox reality series is well worth any initial worries, he added.
"The thing about Idol is, if you're respectful of it, and you enjoy it, and you're a good ambassador for the show, you're in it for life," Johns told EW.