The Nevers is a show full of Joss Whedon's common tropes, held aloft by the strongest actresses he ever got. A show lives or dies by its actors regardless of the pitch. Shows attract fans when they become interested, intrigued, invested in the characters. Once a show has characters who are fan favourites, it becomes a hit. It survives and gets renewed because the audience will come back week after week to watch their favourite characters.
The Nevers is Whedon doing Steampunk, and in a way reclaiming the tropes he introduced in Buffy the Vampire Slayer that has become standard for female-oriented Young adult fantasy sagas. An angst-ridden, unstoppable heroine, her lighter best friend, her mad female nemesis, and a world full of mystery and intrigue. Amalia True (Laura Donnelly) has a mission to protect the Touch, and possibly a deeper mission than that, to fight an oppressive world threat or order. She has a mysterious past as much as a mysterious present. She's just like Buffy and Echo (Eliza Dushku) from Dollhouse. Prudence Adair (Ann Skelly) is like Willow (Alyson Hannigan) and Kaylee (Jewel Staite) from Firefly, the heroine's plucky best friend. Maladie (Amy Manson) is the heroine's crazy pixy nemesis similar to Drusilla (Juliet Landau). River Tam (Summer Glau) is a variation on the crazy pixie dream girl. Olivia Williams plays a similar role in The Nevers that she played in Dollhouse, the older women who might be a mentor or enemy of the heroine.
A show as complex and complicated like The Nevers needs a strong lead, and Donnelly might be the best actress to ever lead a Whedon show. She can suggest volumes of subtext and emotion with just a glance when lesser actors can't manage that with a long speech. Amelia True is a character front-loaded with a mysterious past, a suicide attempt, a rebirth, and a fanatical devotion to a mission. She does not believe she is a good person or heroic or worthy of her mission, but she will not falter from it. She has done terrible things in her past that we don't know about yet. Donnelly makes the viewer feel she's not just competent but unpredictable. For someone who's already established as violent and ruthless, that makes her dangerous and mysterious. The viewer doesn't always know what she's going to do next.
Donnelly has said in interviews that she plays Mrs. True with the subtext of being an Irish woman living under British Colonialism and that translates into the character living under Victorian patriarchal oppression. This lends the show multiple political layers that weren't on the page before she came along. The show has a lead actress as intriguing as the character she plays. And Amy Manson uses her training to lend more gravitas to Maladie's insanity than expected. Ann Skelly also brings multiple layers to Prudence's optimism and religiosity than just the cheerful best friend figure.
Donnelly, Skelly, Manson, Williams, and the rest of the British cast are, as classically trained theatrical actors, the strongest actors a Whedon (and epic fantasy) show could possibly get. They give The Nevers a deeper emotional weight than usual and grounds the Steampunk fantasia in an emotional reality that makes it feel like it matters.