Maggie Smith returns to her stage roots in her new movie The Lady in the Van, reprising the role she played in Alan Bennett's play, based on his real-life experience with Mary Shepherd, a homeless woman who parked her van in his London driveway and stayed for 15 years.

Maggie Smith in The Lady In The VanSmith was comfortable in her new role

And Smith is enjoying the chance to sink her teeth into such a juicy role. "After all these years, I am known more for Harry Potter and Downton Abbey than anything I've ever done," she says. "It goes to show that you have to always be prepared for anything."

Before becoming a household name with Potter and Downton, Smith was already an acclaimed stage and film actress with two Oscars under her belt - for 1968's The Prime of Miss Jean Brodie and 1978's California Suite. She played Professor McGonagall in all eight Harry Potter films, seen by more than a billion people. And she has won two Emmys as Downton's matriarch Lady Violet, a role that comes to an end at Christmas after six high-rated seasons.

Watch the trailer for 'The Lady In The Van' here:

At 80, Smith has learned to keep her roles in perspective. She preferred playing Miss Shepherd to Lady Violet simply because the costumes were more comfortable. "I don't really tend to be close to either role," she said. "I feel easier with The Lady in the Van than that lady with the hat on. It was much easier, as an actor, to be Miss Shepherd because she didn't mind about how she looked. That was such a relief. Lady Violet was forever in those corsets and things that Miss Shepherd would never dream of getting into. I have never been one for dressing up and putting on airs and graces. I would not say that I'd like to live in a van either - my home is my dearest possession - but that character had such a spirit."

More: Read our review of 'The Lady In The Van'

She also enjoyed the chance to play a character she had previously incarnated on stage. "Revisiting the part this time is very different," she says," On stage it's much broader; the film was much more concentrated. It was a whole different thing. The stage is more demanding, but it was a long time ago."

The other main difference was spending all day in an actual van parked on the street where the events really took place. "The van wasn't the most comfortable of places," she says with a wry smile, "but it was where I spent most of the time. It was not very easy being constricted in a van. The rest of the cast were in luxury in a real house."

The veteran actress admits that it was a comfortable change from 'Downton Abbey'.