Harry, who joins Jennifer Lopez and Keith Urban on the reality show, said he thinks "the whole fun of it is not knowing what to expect".
The jazz singer and musician has previously been an Idol mentor.
Jennifer was a judge in 2011 and 2012 while Keith was a judge last season with bickering divas Mariah Carey and Nicki Minaj.
When Jennifer was asked if she has advice for Harry, Keith joked: "Yeah, shut the hell up."
Jennifer also replied "less talking".
The next Idol season begins in January.
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