What exists at the edge of love? If you peer over the precipice, will you find the remains of reason, sanity, trust and grace?

That swan dive to potential ruin is examined in The Edge of Love, a film which explores the real-life relationship of Welsh poet Dylan Thomas (“Brothers & Sisters” star Matthew Rhys), his live-wire wife, Caitlin (Sienna Miller), Thomas’ childhood love, Vera Phillips (Keira Knightley), and her war-hero husband, William Killick (Cillian Murphy), as two couples whose lives became maddeningly intertwined.

Set against the backdrop of London and Wales during the air raids of World War II, Sienna Miller felt the story had an urgency and impulsiveness that fascinated her.

“The way people behaved,” Miller explains, “was largely due to the fact that death was potentially imminent, around any corner you could die. And that’s why I think a lot of these situations occurred, because they were all living with abandon. They were all living on the edge, and that’s mirrored in the title, because of the war.”

Tackling a specific time period and working within the constraints of playing real people is always a steep challenge, but the filmmakers were less concerned about creating a biopic and more interested in investigating these relationships, both romantic and platonic, that went from simmering to eruptive.

“I think love is a really hard thing to define,” Miller says. “It’s multifaceted. What’s interesting about this film is you could put these characters in any era, and their responses to the situations they’re in would be the same. [Love] is a very malleable subject.”

Having delved into the tumultuous, passionate world of the Thomas’, Miller, whose romantic exploits have been extremely well-chronicled, claims she still has no idea what happens along the road to romance, let alone at the edge of it.

“I’m not sure what happens at the edge of love,” she laughs. “I think probably a lack of intelligence.”

The Edge of Love releases in select theaters March 13.