The show is set to open in London next week, with many people eager to see how Harman portrays the infamous 'Mr Nasty'.Speaking to Phillip Schofield and Holly Willoughby, the actor said he enjoyed getting to know Cowell in preparation for his new role."We've done a few press days together and I find him incredibly charming and really easy to hang out with," he said."I was looking for his flaws but he's actually a very nice man, which is disappointing..."He is very much what you see actually and I've sort of highlighted those parts and taken that as far as I can beyond that."I focused on Simon's ego, so I spend a lot of time just loving myself and that seems to work," he joked.I Can't Sing! takes an "affectionate poke" at television shows like the X Factor and includes characters that are very much similar to Cowell, Louis Walsh and Cheryl Cole.The musical is currently previewing in London's West End before its opening night on Wednesday 26 March.Harman explained: "We've been doing about two weeks (of previews) and it's been going really well, we've had standing ovations every night and it's been a bit of a riot really."It's an irreverent mickey-take, that's the best way of putting it."I had people in the other night who said it reminded them of Monty Python, it has elements of the surreal about it... the whole thing is completely mad."So all the people that you know, i.e. Simon and Louis, that's just the starting point and then we just go completely out there.But while some would be worried that Cowell would not take too kindly to such blatant teasing, the music mogul has backed the show.However that's not to say the man portraying the X Factor judge didn't have his doubts as to whether or not he would find it funny."Simon came in to watch and I was nervous because we have pushed the limits of acceptability I suppose... in a comedic fashion."But he came in and gave me the thumbs up. I was worried...we have to make sure that he is happy and he has been brilliant."