The Crown’s Gillian Anderson reveals she was forced to wear fat suit to play Margaret Thatcher in season 4

, Gillian weights around 55kg, five kilograms lighter than the Iron Lady when she came to power.

The Fall star Gillian she and partner Peter Morgan, a writer on The Crown, had outlined some clear boundaries.

She told Harper's Bazaar: "For our own 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!"

In regards to playing a controversial figure like Thatcher, Gillian said she was able to prevent her personal opinions of the former prime minister's policies getting in the way of her portrayal.

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."

The vastly experienced actress has also become very adept at switching in an out of character while on set.

Gillian added: "'I’m pretty good at compartmentalising in my life, period. 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."

Catch season four of The Crown on Netflix from Sunday November 15.