made a departure from typical Hollywood moviemaking for her role as Dr. Eve Saks in Dallas Buyers Club
– her wardrobe came from Goodwill and she went makeup-free.
"I am not someone who is driven by vanity. Even in ways that I should be. I'm not very good at the whole vanity thing," Garner, 41, told PEOPLE at a press day for the movie at the Four Seasons hotel in Los Angeles Monday.
"But I did love that [director] Jean-Marc Vallée came to me and said, 'I don't want you to look pretty.' He almost didn't give me the job because he just assumed . . . and I said, 'I don't care. Never touch my face with makeup. I don't care.' And he didn't. I never sat in the makeup chair."
The film takes place in the late 1980s, and Garner wears clothes from the period, but she was sure to point out that they weren't glamorous vintage pieces, and that her biggest costume concern was being able to breastfeed her son Samuel
, who was an infant at the time.
"The clothes were from Goodwill. It's not like they were from a fancy vintage store or made to look vintage. They were straight up Goodwill, and I had this little baby. So, when I was in costume fittings, it was about, like, 'How can I get to my boobs?' And so I just could have cared less. It was great for me."
The film tells the story of an HIV-positive Texas man
(played by Matthew McConaughey
) who challenges the pharmaceutical companies and helps to provide medication and supplies to people with HIV.
McConaughey lost more than 40 pounds for the role
and Garner said she tried to help him.
"I was there to support him, and Jared came in, and we all kind of went, 'Oh he's doing this. Okay. He's not joking around.'"
Let's start off the movie award season! Watch PEOPLE.com's exclusive live-streaming show with interviews from the red carpet of the Hollywood Film Awards on Monday, Oct. 21, starting at 9 p.m. ET.