Sienna Miller the queen of boho-chic joins dapper Kevin Costner at premiere of his $100m new drama Horizon: An American Saga

Kevin Costner and one of his stars, Sienna Miller, put on a glamorous display as they arrived at the premiere of his new Western film Horizon: An American Epic: Chapter 1.

The 69-year-old director, co-writer and leading man followed up his announcement that he was done with his hit series Yellowstone by turning the attention toward his new multi-part epic.

He was joined at the premiere by several of Horizon's stars, including Sienna Miller, who looked casual yet cool in a lovely white dress.

Despite the high-profile nature of Costner's epic — which he envisions as four films total — the Western is currently feared to flop at the box office after being projected to earn just $12 million at the domestic box office on opening weekend, despite Costner having contributed around $38 million of his own money to the passion project.

Kevin didn't reveal any fears about his film's success at the premiere, as he arrived looking cool and collected in sunglasses and bearing a wide grin. 

Kevin Costner and one of his stars, Sienna Miller, put on a glamorous display as they arrived at the premiere of his new Western film Horizon: An American Epic: Chapter 1

Costner directs, co-writes and stars in the first of his planned four-part Western epic 

The JFK star was dressed appropriately for the premiere of a Western with a smart beige linen suit.

He wore it with a classic white shirt left unbuttoned and tie free to match the casual vibe, and with a set of square-toed brown Chelsea boots.

He blocked out the setting sun with a pair of dark tortoiseshell sunglasses, though he took them off once he was out of the direct sunlight. 

Sienna showed off some Western chic of her own with a low-cut white spaghetti-strap dress with delicate lace over her bust.

She included some touches of the boho-chic style she's known for with a thick banana-shaped gold necklace and a gold bracelet with colorful beads and chains dangling from it.

Also making a splash at the premiere were Sam Worthington and his model wife Lara Worthington. 

He was dressed in a black suit and wore a matching black shirt, along with patent leather shoes

Lara looked lovely in a pale green dress with a ruffled sheer skirt, which she paired with a similarly shiny set of pointy-toed boots.

The JFK star was dressed appropriately for the premiere of a Western with a smart beige linen suit

He wore it with a classic white shirt left unbuttoned and tie free to match the casual vibe, and with a set of square-toed brown Chelsea boots

He blocked out the setting sun with a pair of dark tortoiseshell sunglasses, though he took them off once he was out of the direct sunlight

Sienna showed off some Western chic of her own with a low-cut white spaghetti-strap dress with delicate lace over her bust

She included some touches of the boho-chic style she's known for with a thick banana-shaped gold necklace and a gold bracelet with colorful beads and chains dangling from it

Also making a splash at the premiere were Sam Worthington and his model wife Lara Worthington. He was dressed in all black with patent leather shoes, while she wore a pale green dress with a ruffled sheer skirt and similarly shiny boots

Abbey Lee, who stars with Costner in the first Horizon film, put on a striking display in a sleek black off-the-shoulder dress with a neckline decorate with feathers that rose up to her chin.

The sleek dress featured a tall slit to showcase her trim legs, and she completed her outfit with black open-toe heels that strapped around her ankles.

Jena Malone also opted for a black off-the-shoulder dress.

She glowed in an item decorated with a grid of tiny rhinestones, and her plunging neckline was held together with a jeweled clasp.

Jena posed on the red carpet with Ella Hunt, who looked glamorous in a single-shoulder beige dress with a diagonal cut-out over her chest. 

The dress shimmering thanks to rows of golden sequins running down it.

Maya Erskine hit the red carpet in a cream-colored skirt and a matching long-sleeve top tied at the top and left open to put the spotlight on her burgeoning baby bump.

She was joined by her husband Michael Angarano, who wore a semi-sheer black button-up shirt with a tear-drop pattern, along with black slacks.

Abbey Lee, who stars with Costner, was dressed in a black off-the-shoulder dress with a neckline of tall flowers and a sky-high slit. Jena Malone also favored a black off-the-shoulder dress, though hers was decorated with a grid of sparkling rhinestones

Jena posed on the red carpet with Ella Hunt, who looked glamorous in a single-shoulder beige dress with a diagonal cut-out over her chest. The dress shimmering thanks to rows of golden sequins running down it

Maya Erskine hit the red carpet in a cream-colored skirt and a matching long-sleeve top tied at the top and left open to put the spotlight on her burgeoning baby bump

She was joined by her husband Michael Angarano, who wore a semi-sheer black button-up shirt with a tear-drop pattern, along with black slacks

Isabelle Fuhrman looked glamorous in a sleeveless violet satin dress with a voluminous skirt and a powder blue sash tied around her midriff. 

The actress and model Wasé Chief showcased her slim figure with a sheer bodice featuring silvery strips, which she wore with a classic black skirt. 

Jamie Campbell Bower, who has a potentially deadly confrontation with Costner's character in the trailer to Horizon, arrived in a well-fitted charcoal three-piece suit decorated with pinstripes, which he paired with black boots.

Billy Zane flashed a winning grin when he arrived on the red carpet in a navy blazer that he wore with a partially unbuttoned white shirt and pale pink trousers.

Michael Rooker also kept his look casual. He wore a short-sleeve gray patterned shirt with black trousers and shiny black shoes. 

Luke Wilson stuck to a classic navy blue suit with a matching tie and black boots.

Giovanni kept his look low key with a midnight blue suit. 

He paired the nearly black outfit with a similarly dark shirt and a black-and-blue striped tie. 

The magnificently maned Jeff Fahey rocked a black double-breasted blazer with peaked lapels, along with casual shirt, dark pants and black suede boots. 

Adam Carolla arrived in a gray suit with chunky black glasses and black boots.

Isabelle Fuhrman looked glamorous in a sleeveless violet satin dress with a voluminous skirt and a powder blue sash tied around her midriff. The actress and model Wasé Chief showcased her slim figure with a sheer bodice featuring silvery strips, which she wore with a classic black skirt

Jamie Campbell Bower, who has a potentially deadly confrontation with Costner's character in the trailer to Horizon, arrived in a well-fitted charcoal three-piece suit decorated with pinstripes, which he paired with black boots

Billy Zane flashed a winning grin when he arrived on the red carpet in a navy blazer that he wore with a partially unbuttoned white shirt and pale pink trousers

Luke Wilson stuck to a classic navy blue suit with a matching tie and black boots

Giovanni kept his look low key with a midnight blue suit. He paired the nearly black outfit with a similarly dark shirt and a black-and-blue striped tie

The magnificently maned Jeff Fahey rocked a black double-breasted blazer with peaked lapels, along with casual shirt, dark pants and black suede boots. Adam Carolla arrived in a gray suit with chunky black glasses and black boots

Tim Guinee leaned in on western clothing with a purple purple patterned waistcoat under his blue suit jacket. He wore them over a white patterned shirt and completed the look with dark beige jeans and black boots

Alejandro Edda looked cool in a striking red corduroy three-piece suit with an unusually shaped outer jacket

Tatanka Means, who previously appeared in Martin Scorsese's Western crime epic Killers Of The Flower Moon, stood out in a sleek black Louis Vuitton suit with peaked lapels

Tim Guinee leaned in on western clothing with a purple purple patterned waistcoat under his blue suit jacket. 

He wore them over a white patterned shirt and completed the look with dark beige jeans and black boots. 

Alejandro Edda looked cool in a striking red corduroy three-piece suit with an unusually shaped outer jacket. 

Tatanka Means, who previously appeared in Martin Scorsese's Western crime epic Killers Of The Flower Moon, stood out in a sleek black Louis Vuitton suit with peaked lapels.