Elizabeth Banks had to dig deep for her latest role as Frankie, an alcoholic single mother in People Like Us, out Friday.

Unknown to her, Frankie is the half-sister to Sam, played by Star Trek hunk Chris Pine. Sam finds out that he has a bigger family when his late father, a philandering record producer, leaves him a note, along with a load of cash to deliver to Frankie, the product of a one-night stand.

When he meets up with Banks’ character at an AA meeting, Sam doesn’t reveal his identity straight away — he kind of needs the money himself and may want to see if his newfound flesh and blood is worthy of it.

This leads to an unusual dance between a man who knows the score and a vulnerable bartender who thinks she may have found a great boyfriend.

“Frankie has a lot of walls of ice around her heart, and Sam breaks away at that,’’ said Banks on a press stop at the Four Seasons Hotel Miami. “The great thing she realizes is that she is open to the love business, besides the fact that he is her brother. Relationships are not something she’s been very good at in the past. Loving and being loved become really possible for her, and she had forgotten that.”

The movie’s universal message — the importance of family — kind of sneaks up on you. The Hunger Games star, 38, can relate.

“I’ve heard a lot of people say the first thing they want to do when they see this movie is call somebody, whether it’s their sister or brother or mother or father,’’ says Banks, the oldest of four kids. “In my opinion, it’s about the damage that gets done and the dysfunction of families. But really, we’re all doing the best we can.”

Banks, better known for romantic comedies like Zach and Miri Make a Porno, Definitely, Maybe, The 40-Year-Old Virgin and Our Idiot Brother, was hooked by the drama of People Like Us.

“I just got pulled in right away. I felt like I understood Frankie and recognized her and wanted to be her,’’ she said. “She’s a survivor and has so much heart. She’s faced so much and is still standing, putting one foot in front of the other. I really admire that quality in people.”

Despite a nasty blowout after the big reveal, the film ends on an up note.

“We come to realize our father loved us, and that’s all that really matters,’’ said the Massachusetts native, 38. “Your past doesn’t have to determine your future.”

But no one said it was a breeze to tap into the dark side.

“It was really hard,’’ she admits. “I knew I would have to go some really emotional, deep places. This character is very much lied to. It’s devastating when someone we really trusted is someone who turns out to have betrayed you.”

Personally, Banks is in a better place. She’s happily married to her college sweetheart Max Handelman; they have a son Felix, born March 2011.

Like any new mom, Banks was lacking in sleep. Not that you’d know that.

“That’s all thanks due to my makeup artist — and great lighting.”