A new skin to be in

British actress Emily Mortimer always keeps audiences guessing by making certain she doesn’t repeat herself onscreen. In the last few years, she’s brought to life such diverse characters as an insecure struggling actress in the 2002 comedy drama “Lovely & Amazing,” for which she won an Independent Spirit Award, and a hedonistic British society girl from the 1920s in “Bright Young Things.” In her latest film, “Dear Frankie,” which opened Friday, the 33-year-old Mortimer plays a poor single mother in a seaside Scottish town who persuades a stranger (Gerard Butler) to pretend to be her deaf son’s long-absent father.

Mortimer’s father is writer John Mortimer of “Rumpole of the Bailey” fame, which may account for her education: St. Paul’s Girls School in London, Lincoln College, Oxford. She made her film debut in 1996’s “The Ghost and the Darkness” and, in addition to her indie work, she’s appeared in such films as “Scream 3” and “Notting Hill.” She and her husband, actor Alessandro Nivola, are the parents of 18-month-old Sam.

Does “Dear Frankie” mark the first time you played a mother on film?

Yes, it was. I was very nervous about playing a mother without having been one. I think it’s almost impossible to imagine unless you have done it. As an actor, you can imagine most things. You can imagine what it might be like to take a lot of drugs or rob a bank or even kill someone in certain instances, but to be somebody’s mother!

But you were pregnant while making the movie.

I was grateful for the fact I was pregnant. There are two sort of defining emotions that come with parenting. One is this totally baffling love that comes from nowhere and is completely unexpected and that protective feeling that you are totally responsible for this person’s life. That happens when you are pregnant. The other emotion is just the guilt, the feeling that you are not quite doing it right, or worrying you are not quite doing it right. Those two pulls are very well drawn in the film, and you definitely get that from the minute you are pregnant -- you start worrying if you are doing it right.

Did you get the opportunity to spend time with Jack McElhone, who plays your son in the film, before production began?

We had this sort of disastrous first date where his mum forced him -- poor thing -- to take me to some art gallery in Glasgow. We walked around and had a cup of tea in the coffee shop and sat there. I never felt older in my life.

You also play Inspector Clouseau’s secretary in the upcoming “The Birth of the Pink Panther” with Steve Martin. Is your character as clumsy as the good inspector?

I am very organized and perky and French, but the minute I am in a room with him, all chaos ensues.

You know that old adage that drama is easy and comedy is difficult. Was it especially difficult to do slapstick?

I got real nervous [before production]. What is the clue to this? It does seem like a totally different discipline in a way. I never quite worked out the right answer, apart from just totally losing my inhibitions and sort of allowing myself to just completely go for broke.

You always mix small films with mainstream Hollywood films. Is that a conscious effort on your part to find a balance between the indie and studio film worlds?

It is not so much a conscious choice to do smaller and bigger films as a choice to constantly do different things and keep yourself and everyone else from getting bored. Also, I think all the roles I have done have been fairly different, and I try to keep it that way. I think it’s something to do with having a kind of a privileged, middle-class upbringing and feeling that in a way, my life up until when I started acting was kind of on a predictable trajectory of sort of going to certain schools....

Being easily defined in that way is kind of scary. So it is nice to surprise people and surprise yourself and find these totally different personalities within yourself.

What was your father’s reaction when you told him you wanted to be an actress?

He was thrilled. I think he would have loved to have [been an actor]. He would have liked to have been Fred Astaire and sauntered down a white staircase with a silver top cane and a top hat. So he totally approved.

Would you and your husband like to work together?

We would love to do something together. I so admire him as an actor. I would just love the chance to work with him, but it’s hard. I think falling in love onscreen if you have already been together for five years can be slightly dangerous territory because what if people think you have no chemistry? It would be terribly depressing!