The Sex Education star said the show will get under the skin of the former prime minister, capturing the “human being” behind the notoriously tough politician.
Gillian will debut as Thatcher in the fourth series of the hit Netflix drama, which is released later this month.
Speaking to Harper’s Bazaar, who named the actor among their women of the year, about the role, she said: “I had to get to a point where it’s nothing to do with my opinions of her policies, of her actions.
“It is only about her as a human being and her motivation as a politician and as a mother.”
Gillian also discussed working with her partner Peter Morgan, who created the series.
She said: “For our sanity, and actually for the benefit of the relationship, we had very clear boundaries: ‘I am not going to comment on the script, but you are not allowed to comment on the performance.’”
The former star of The X-Files said she is good at “compartmentalising” her life and is able to switch off from her roles.
“I think I learnt that quite young, being a young mother, in a really intense TV series where it was either full on on-set, or I was in my trailer having to shut the door and no longer be that person but be mum,” she said.
Gillian is joining the cast of The Crown for the fourth series alongside Emma Corrin as Princess Diana.
Meanwhile, it has been confirmed that for the fifth series, Imelda Staunton will take over the throne from Olivia, while Jonathan Pryce will succeed Tobias as Prince Philip.
Lesley Manville and Elizabeth Debicki will be playing Princess Margaret and Princess Diana, although it’s yet to be confirmed who will be appearing opposite her as Prince Charles.
The December issue of Harper’s Bazaar is on sale from Thursday, while The Crown’s fourth series will debut on Netflix on 15 November.