When TV and movies do flashbacks, they face a dilemma: They can either cast a younger actor that looks like their star, sacrificing easy recognition OR they can just slap a wig on their actor as an indication that we're seeing the past. And it always looks wrong. It's the uncanny valley of hair.
It's fake hair on a real person, and it shatters the illusion that we're watching something real. So, unless humor is intended, it doesn't work.
There are a number of reasons for creating definite "in the past" hair. You know immediately you're in a flashback. Sometimes, it plays on certain stereotypes to give viewers instant knowledge about what the person was like back then. Sometimes, it's just for fun.
In some cases, you've got immortal beings or reincarnations or doppelgangers or whatever and the writers take that as license to travel throughout time, dumping their characters wherever the mood strikes them. And that means putting modern actors in period hair for tiny fractions of the whole arc. That's when things get magical.
Here are the best and worst kinds of "in the past" hair. Spoilers!
Long Hair Means I Used to Be an Innocent
If anyone is the patron saint of this look, it's Veronica Mars. Long locks are the province of Disney Princesses and their particular brand of naive goodness. So if a female character's flashback wig is long and her normal hairstyle is short and spunky, you know something happened to make them cut their hair and pick up a new persona.
See also: The Huntress on the short-lived Birds of Prey, Shannon on Lost (God, that's a bad wig.)
Image from: vm-caps
I Was a Teenage Goth
This is the flip version of the above one. Why was everyone in television a teenage goth? Is it the moment of time we live in, where we remember the whole wave of goth teen panic, but there is no quicker way of establishing a character's past than showing them decked out in black cloths, lipstick, and dyed hair. It's a short hand for seeing a character who is all nice and normal in the present had a past as a rebel. Maybe it's that you can cover your actor in clothes and makeup and have an excuse.
Image from: How I Met Your Mother Wiki
Messy Hair, Messy Life
Every Arrow flashback to before the island is hilarious. Stephen Amell loves wandering through those as a rich idiot (there was even a flashback in season 2 where he basically showed up behind Moira and whined "Mooooom, I need money for the pizza guy!" with exquisite entitlement). It's not enough that he acts differently, he also has hair which shows how not disciplined he was. His longer hair is his douche hair. No one wondered why he came off the island with less hair than he got there with? Where did he get a razor?
See How I Met Your Mother's flashbacks (again), Huck's homeless hair in Scandal (he also has close-cropped hair when he gets it together), the endless joy that is Lost's Jack Sheppard flashbacks (He and Ollie should have a club)
I Wasn't Always Bald
This one's just easy. If a character is bald in the "present," then flashbacks will give them hair. Or, a bad toupee, Locke.
Not quite the same as showing the same character in the past, this one's the practice of just having people's ancestors look exactly like them. Just, you know, with ridiculous hair. Back to the Future did it for laughs, but when it's used in a drama, it loses something. Yes, some people do look a lot like their parents. But not to the extent fiction would have you believe. It's less like having parents than clones. But, cheaper than getting another actor, I guess.
See also: JAG had Harm and his father, The Pretender has Miss Parker and her mother,
The Many Hairstyles of the Immortal
The other categories are all well and good, but this? These are the most magical of past wigs. At least these characters aren't supposed to age, so they can get away with this method. And they live forever, so we love to see them in different eras. And different eras mean truly AMAZING hair. Highlander, you're up:
Just sitting here, chanting "Methos as Death hair, Methos as Death hair!" You tell us, are these the Four Horsemen of the Apocalypse or a hair band?
The above and so many. So, so many.
But there's no way the vampires let the immortals take this one.
Spike. Spike wins this round for the vampires.
Once Upon a Time kind of straddles this (since they are flashbacks) and the "alternate self" trope (since it's a different world.) But there are some epic flashback hair going on in this show. Did anyone else laugh out loud last season, when returning home instantly gave Snow White back her ridiculously long (and non-Disney version) princess hair?
No real category for this, but what on earth was going in this episode of Desperate Housewives?
And Friends took great joy in embarrassing its cast for flashbacks:
In conclusion, the "winners" of this are definitely Lost (it was a show premised on flashbacks, how are the wigs SO BAD?), Highlander (METHOS' DEATH HAIR), and Buffy/Angel taking it for Team Vampire. Once Upon a Time is disqualified for the performance enhancing effect of being fairy tales trapped in our world, and therefore getting the alternate universe hair bump. Cheaters.