The "Aviator" star plays the late legend in "The Secret Life of Marilyn Monroe", which is based on J Randy Taraborrelli's book of the same name, reported Us magazine.
"I was extremely apprehensive; it's Marilyn Monroe. Those are really, really big shoes and I wasn't sure if I could pull it off," she said.
Garner, 31, explains she eventually agreed to take on the part because the TV series examines Monroe's struggles with mental illness, a subject matter the actress was fascinated by because of her own experiences in therapy.
"One of the most favourite things is this (scene) that we used to tell a story and it starts with her sitting down with this fictional therapist and I found that fascinating. I think it's important to talk about mental health and I've been in therapy myself and be able to play such an icon in such intimate sounding board moments of trying to figure out what's going on was really fascinating to me.