The comedian joined the panel alongside Cowell, Amanda Holden and fellow newcomer Alesha Dixon for the new series of the ITV1 talent competition, which kicked off last weekend.
"Simon is the King and I'm the court jester," Walliams told the new issue of Harper's Bazaar UK. "I asked the show's producers, 'Can I make jokes about Simon? About him being camp? About being friends with Sinitta? About his hair?' And they said, 'No, no, no'.
"Then the first act came on and said, 'My dream is to perform in front of the Queen', and I said, 'Well, he's right here - go ahead'."
Walliams admitted that he will reign himself in for the live shows, adding: "The innuendo-meter is definitely a problem. I'm going to have to have a second's delay in my brain."
"For me there's a kind of manic depression that happens where there's been a manic phase of creativity or performing," he said. "I mean, even just being in front of the audience of Britain's Got Talent, I do have this manic energy, and that has to be counter-balanced with something...That's just the way things are."
The full feature appears in the May issue of Harper's Bazaar UK on sale Monday, April 2.
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