From the first Shakespearean performer ever cast as Ophelia, wigs have been a necessary staple in the toolbox of props that aid an actor in assuming a character. They’ve allowed Elizabeth Taylor her austere and almost incomprehensibly silky Cleopatra hair, Cher her dowdiness and her dazzle in “Moonstruck,” and Steve Van Zandt a screen career that doesn’t require his E Street Band do-rag.

But as with all costumes, some are better than others. The great wigs are barely detectable; the not-great ones distracting or, worse, laughable. And now that high-definition cameras can bring out every flaw, the wrong wig can be a constant, unwelcome reminder that the actor is, well, an actor.

Many reasons

It’s not always easy to discern why a wig is necessary. Sometimes it has to do with the actor’s earlier or concurrent commitments. Morena Baccarin wore one for her role as the wife of a returning Marine in the first episodes of “Homeland” on Showtime. She has said she didn’t have time to grow out her hair after playing an alien on the short-lived revival of the series “V.” By Episode 5 of “Homeland” the wig was gone and the character even mentioned her “haircut” on screen. Conversely, for the coming Starz show “Magic City,” in which four of the five main actresses are wigged, Olga Kurylenko needed shorter hair for her character, the wife of a hotel owner. However, Beatrice De Alba, the series’ department head hairstylist, said Kurylenko had a Pantene contract in Eastern Europe requiring her to keep her hair long.

In some cases a wig is worn simply because the actor’s hair can’t be styled to resemble what the producers had in mind. In “My Week With Marilyn” (2011) Michelle Williams’ locks were far too short, said the makeup designer Jenny Shircore, who oversaw hair for the movie. On NBC’s “Are You There, Chelsea?” Chelsea Handler wears a brown wig because she’s playing her own real-life sister, a brunette, said her hairstylist, Gina Monaci.

Wigs also cut down on continuity errors. In “One for the Money” this year it was hard to maintain Katherine Heigl’s hair as curly and disheveled as the description of the character, Stephanie Plum, demanded. “Stephanie Plum is not a hair girl,” said Sean Flanigan, who styled Heigl’s wig. “It was easier to keep a wig than constantly trying to curl her hair exactly the same way every day.”

And sometimes a wig is protective, saving an actress from the damage that constant styling can inflict. Julianna Margulies’ wig on “The Good Wife,” on CBS, allows her character, Alicia, to have straight hair without subjecting Margulies’ own curly locks to sustained heat and hairstyling products. “A lot of times actresses don’t want their hair done every day, because it takes a toll,” said Carl Bailey, department head hair stylist for “Revenge.” “Women with curly hair, also black women, use wigs because their hair is fragile.”

Sometimes a wig is needed because a character has to look different over the course of a show’s season. Speaking of “Revenge,” the duplicitous character played by Emily VanCamp wears wigs as disguises and in frequent flashbacks. These wigs are obvious, and it could be because we know she’s in disguise. But it could also be because VanCamp isn’t very comfortable in them.

Bailey said VanCamp didn’t love the wigs: “One is so dark that it photographs black — not a very becoming look for her.”

(Incidentally, it isn’t as if VanCamp were going au naturel when she isn’t wigged. Clip-on extensions allow her hair to “grow” as the season goes on. But extensions are a whole other subject. Bailey estimates that 75 percent of all women on TV, more in film, wear hairpieces. If you’re distressed that your hair seems far more limp and sparse than the people on screen, rest assured: It isn’t.)

Realism

Make no mistake. VanCamp’s wigs aren’t obvious because they’re lacking in quality. Like Margulies’ wig, and like those for “One for the Money” and “My Week With Marilyn,” they are lace-front — the gold standard for wigs — that is, the result of weeks of painstaking handwork by wigmakers who thread and knot strands of hair, one to three at a time, through pieces of lace with a crochet hook. Each is inserted with the hair cuticle going in the same direction, the way it’s arranged naturally on heads, so that the hair doesn’t tangle. Such wigs can cost $4,000 (“My Week With Marilyn”) to $10,000 (“Are You There, Chelsea?”).

But a wig’s success isn’t just about the craft, it’s also about the application. “If you wrap a head and put all the hair on top, you’ll look like a Conehead,” De Alba said. “You pin hair down in a certain direction, put on a fine netting and wrap the head so tightly with flat pins that it looks like the actor is a bald person.”

“The younger generation is really starting to accept them as an accessory,” said Linda D. Flowers, department head hairstylist for “The Hunger Games.” She said this started with “Hannah Montana,” the Disney Channel show about a teenager who wears a wig when she’s in her pop-star guise.