Ryan Gosling, 41, appears in the film alongside Margot Robbie, and his long-time partner has embraced his movie makeover, which includes his eye-catching blonde hair.
The insider said: “They were actually having fun role-playing together after he went blonde. They both got some good laughs.”
The loved-up couple has been together for more than a decade, and their chemistry and affection are said to remain “off the charts.”
Mendes has taken a step back from her acting career since having her first child.
But she previously revealed that her “ambition is coming back.”
Mendes was one of the most in-demand actresses in Hollywood at the height of her career. But in the early days, the 48-year-old actress was told that she was “too ethnic” by one casting director.