The pair are set to return alongside Amanda Holden and Alesha Dixon for the eighth series of the popular talent contest on Saturday.But Cowell said being reunited with Walliams was not as exciting as it should be."He's really got annoying this year," Cowell explained. "He was kind of amusing before but God forbid I make the audience laugh more than he does - he gets quite dark."Seriously, I was thinking about it when I was working with him. It's like having a child who's suddenly turned into a giant at the age of 11."He is honestly mentally an 11-year-old in this huge body. He has got a kind side but he can be very, very annoying to work with."The music mogul added Walliams was too quick to jump into his 'head judge' chair when he had to make a dash across the Atlantic during this year's auditions in order to make it to New York in time for the birth of his son Eric."He couldn't get into that seat quick enough!" Cowell said."He was praying for (girlfriend) Lauren to give birth early. In fact I'm sure he called her up saying, "Push! Push!" because as soon as I was out he was in that chair, but it's only going to happen once trust me!"Britain's Got Talent returns on Saturday, and Cowell said there was plenty of amazing, and amazingly terrible, acts on offer."We have found these incredible motorbike stunt riders. I'm amazed that half of them didn't get killed...I love them. They're like modern day Evel Knievels."We also found a dancing owl that didn't dance, so no fear of that winning."The one I'm dreading winning is a fashion designer, who in my opinion is the worst fashion designer of all time -and somehow he's made it through to the finals because David, Amanda and Alesha are on the judging panel."I swear to God I don't know what they're seeing because I'm seeing something very different."Britain's Got Talent returns to UTV on Saturday 12 April from 7.15pm.