"I was really lucky," McGregor said. "I mean, there are some people in his world, who kindly sat down with me, people he worked with. But, you know, the most amazing friendship he had was with Liza Minnelli. And they had a really beautiful best friendship."
"I wanted to meet her anyway and reassure her that he was in good hands, he was in safe hands with me," he added. "That I wanted her to know that I respected their friendship, and I understood that he was very important to her."
McGregor said Minnelli expressed her "great love" for Halston during their conversation.
"She asked me to keep our conversation private, which I would always respect, but I got -- what she gave to me was the great love she had for him, and I could see through her, you know, the great love he had for her," the actor said.
"It's a responsibility, you know, when you're playing somebody who's lived and to get him right. And I was lucky that she was able to share some of her memories of him with me," he added.
Halston, born Roy Halston Frowick, came to fame as a designer in the 1970s. He died of AIDS-related cancer at age 57 in March 1990.
Halston is based on the 1991 book Simply Halston by Steven Gaines. The series co-stars Krysta Rodriguez as Minnelli, Rebecca Dayan as Elsa Peretti, David Pittu as Joe Eula and Bill Pullman as David J. Mahoney, and started streaming this month.