The 47 year old plays sinister wizard Lucius Malfoy in the franchise and the Brit was so sure of the style he wanted the villain to adopt, he approached the films' costume department with his ideas.
But it was Radcliffe who convinced director Chris Columbus to approve the outfits.
Isaacs explains, "When I first got there they had done drawings of what Lucius would look like - a pin-stripe suit and salt-and-pepper hair and I thought, 'This is the one crack (chance) I'll get in life to play a wizard,' so I begged them for a wig and velvet and luckily they went along with it.
"We went down on set to show the director and he did a double take at me and they said, 'It's Jason, Lucius Malfoy', and he said, 'We've done drawings of you', and I said, 'I want to look like a wizard... can I have a cane or walking stick, I can get the wand out of it', and they said, 'We can't do that, wands come out of people's sleeves'.
"Daniel, who was standing there, said, 'I think it'd be really cool!' and Chris said, 'Alright, let's do that then.'"