Show hosts Ant and Dec revealed that the pair have a "weird" rapport while filming.
"David is inappropriate towards Simon," Ant McPartlin told The Sun. "He flirts with him every day.
"He's constantly stroking him and giving him little massages. There's this weird Simon-David relationship going on since the auditions.
"Simon gets completely wound up by it but it's great. He enjoys it secretly."
Walliams recently revealed that he felt he has "really gelled" with Cowell since joining the panel.
> Simon Cowell promises 'cooler' Britain's Got Talent
Watch ITV's new Britain's Got Talent trailer below: