Jason Isaacs — who plays white-haired wizard Lucius Malfoy in the Harry Potter films as well as Michael Caffee on Showtime’s gritty drama Brotherhood — will let his fanboy flag fly when he guest stars on the upcoming season of Entourage. “I couldn’t be more excited,” he told “I have seen every episode. I can’t tell anyone what I am playing. I can only say I am not playing myself. Christ, I hope no one thinks it’s me. It’s quite a character. I wouldn’t want people thinking he’s based on my personality!”

Isaacs admitted he got a little star struck while taping the series, which returns Sept. 7. “Even after hanging around with these boys for a few days and playing golf with them, I found it hard to shake off my whole fan geekiness,” said Isaacs, who added that he is grateful to Showtime for allowing him the opportunity. “They don’t normally give up their series’ actors [to appear on] any HBO show. But I am such a huge fan of the show and they made the exception for me.”

He also said that he has “no idea what is in store for the third season of Brotherhood. “The creator just had a baby so I wonder if it is going to turn all soft and cuddly. I doubt it strongly. He’s probably going to be more pissed off and grumpy because of lack of sleep so he’ll come to work and take it out on these characters.” Brotherhood returns to Showtime on Nov. 2. “No one has a good time in Brotherhood,” Isaacs continued. “I think that’s why people like watching it because by comparison everyone’s lives are just peachy.”

Next up for Isaacs: the seventh and eighth installments of the Harry Potter franchise. “I wasn’t in six so it has been a long hiatus for me and I can’t wait to get back in that world,” said Isaacs. “It is easy to get back into the mind of Lucius. I just put on the wig. The thing about that wig is that it is long and straight and the only way to get it straight is to tilt my head back and then I’m forced to look down my nose at people. And suddenly Lucius is back in town.”

– With reporting from Carrie Bell