Lightning always strikes twice.

That must be what American television executives believe - that if a show was popular once, surely it can be a hit again.

Hence the constant parade in recent years of revivals, with the original cast intact, and reboots, where new actors replace the old cast.

Sci-fi horror series The X-Files returned for a 10th season last year - 14 years after the ninth season - and has been picked up for an 11th season, slated to premiere in January.

Sweetly kooky family drama Gilmore Girls returned for a four-episode mini-series in November last year, after wrapping up its seventh season in 2007.

Haunting mystery drama Twin Peaks returned in May for a limited series of 18 episodes after two seasons (1990 to 1991) and a movie, Fire Walk With Me (1992).

Kicking off the latest crop of remakes/sequels are 1980s soap opera Dynasty, gay-man-straight-woman sitcom Will & Grace and cranky comedy Curb Your Enthusiasm. Blue-collar comedy Roseanne, witches-as-heroines fantasy Charmed and stylish cop drama Miami Vice are in the works.

Season 10 of The X-Files attracted an average of 13.6 million viewers in the United States, more than for Season Nine. It was enough to justify another season of the paranormal drama, but are reboots and revivals a sure thing with audiences?

In the first place, there are plenty of valid reasons why a show ended. TV executives are not exactly in the habit of killing geese that lay golden eggs; they are more likely to flog a dead horse.

Declining viewership is often a clear sign that viewers are getting tired of a series. Dynasty was ranked No. 1 in the 1984 to 1985 TV season, but by Season Nine, the tired plot twists and silly shenanigans saw it plunging to No. 69.

At its peak, in its fifth season from 1997 to 1998, The X-Files drew an average of 19.8 million viewers. That figure plummeted to 9.1 million in its ninth season from 2001 to 2002.

Declining ratings are also often a result of a programme running out of steam creatively.

One of the most notorious examples of this is in the fifth season of sitcom Happy Days (1974 to 1984), when cool guy The Fonz jumps over a shark while on water-skis. The phrase "jump the shark" now points to the moment a show goes downhill or veers off-course.

Gilmore Girls did indeed jump the shark. Fans had loved it for its rapid-fire repartee, small-town charm and warm and strong mother-daughter relationship between Lorelai (Lauren Graham) and Rory (Alexis Bledel).

For some fans, things soured when Rory went off to college in Season Four; for others, Season Seven was awful as it was the first time that creator Amy Sherman-Palladino was not involved in the show, as negotiations on a new contract broke down when it moved to a new network.

A fan posted in a forum: "S7 was a blatant disregard for everything that made Gilmore Girls great in the first place."

But after enough time has passed, the rose-tinted glasses of nostalgia can make everything seem better in hindsight, as well as make executives greenlight reboots and revivals.

Family sitcom Full House (1987 to 1995) returned for a sequel, Fuller House, last year, with most of the ensemble cast reprising their roles - and was promptly pelted with criticism.

That clearly announced to TV honchos that they cannot bank on fans' nostalgia to sustain a revival.

Fuller House has a "rotten" rating of 32 per cent on the aggregator website Rotten Tomatoes, while TV.com called it "sugary garbage that might drive you to madness if you watch enough of it, which is to say it honours its predecessor... incredibly well".

The sneaky suspicion is that these sequels keep happening because TV executives are lazy or desperate.

When a reboot of Charmed was first proposed in 2013, actress Rose McGowan, one of the original's stars, tweeted: "They really are running out of ideas in Hollywood".

There are revivals that have fared better, perhaps because there are stronger imperatives for their existence.

In Gilmore Girls: A Year In The Life (2017), Sherman-Palladino returned to reclaim her show, now funded by Netflix, and take it forward on her own terms. Its Rotten Tomatoes score is a "fresh" 85 per cent.

American magazine The Atlantic said: "Considering it could merely exist as a cheap cash-in, that A Year In The Life feels so emotionally resonant is somewhat miraculous."

But it also added that the show "won't necessarily convert new viewers - like any revival, it's making a play for a loyal fanbase".

So, in its case, nostalgia works.

The big questions hanging over each remake is whether creatively, there is still something worthy to say, or whether the show's premise is doomed to be stuck in the past. And can it appeal to new audiences beyond the flock of the faithful?

Though Will & Grace was criticised for keeping Will pretty much asexual and also pandering to stereotypes with the role of flamboyant Jack, it was nevertheless ground-breaking for its time for putting a gay man front and centre. But is it still relevant now that Modern Family (2009 to present) features a gay couple with an adopted daughter?

Perhaps somewhat surprisingly, the answer is yes. Will & Grace's revival has a score of 73 on website Metacritic, indicating generally favourable reviews which have singled out the chemistry of the cast and praised it for being familiar yet contemporary.

Web magazine Slate pointed out: "In 2017, Will & Grace is still a show that America needs, because now it's a sharp comedy about white, moneyed, liberal hypocrisy."

One way of reaching out to fans of a show without being overburdened by expectations is to start with a clean slate where the cast is concerned.

For Dynasty, this was probably inevitable, given that John Forsythe died in 2010 and catfighting stars Joan Collins and Linda Evans are now 84 and 74 respectively. The reboot's sexy, younger and presumably fighting-fit line-up includes Grant Show, Nathalie Kelley and Elizabeth Gillies.

Early word has been mixed, with the show scoring a middling 56 on Metacritic from five reviews. Trade rag Variety sniffed that the "whole enterprise reeks of mere adequacy", while the New York daily Newsday said: "Updated, sharply written, socially conscious, this new version wants to improve on the original and often does.

"But what's missing is a compelling reason for a reboot in the first place."

If the reasons are compelling enough for a reboot or revival, who knows, lightning may well strike again.

New crop of revivals and reboots

Revivals are series brought back with the original cast intact. Reboots keep the title, but feature a new cast.

REVIVALS WILL & GRACE

Original run: 1998 to 2006

A sitcom about two best friends, gay lawyer Will Truman (Eric McCormack) and straight interior designer Grace Adler (Debra Messing). Flamboyant Jack McFarland (Sean Hayes) and boozy socialite Karen Walker (Megan Mullally) were the scene-stealing sidekicks.

In the Season Eight finale in 2006, Grace has a baby girl and Will has a baby boy. The two have a fallout that lasts for years, but eventually make up when their children meet in college.

The first episode of Season Nine, which aired in the United States on Sept 28, is titled 11 Years Later, with Will and Grace once again living together in his apartment after their respective divorces.

ROSEANNE

Original run: 1988 to 1997

Roseanne Barr is the sarcastic star of this sitcom about a blue-collar family. She plays Roseanne Conner, John Goodman plays her husband Dan and their children include Becky (Lecy Goranson and then Sarah Chalke) and Darlene (Sara Gilbert).

The Season Nine finale in 1997 had a major twist as it was revealed that the entire show was a story written by Roseanne Conner about her life - with embellishments. So the Conners did not win the lottery and Dan's heart attack near the end of Season Eight was fatal.

Season 10 is slated to air as a mid-season replacement show in the USnext year.

CURB YOUR ENTHUSIASM

Original run: 2000 to 2011

Comedian Larry David plays a fictional, neurotic and cranky version of himself who often ends up in awkward situations, no thanks to his stubborn streak. The supporting cast includes Jeff Garlin as his close friend and Cheryl Hines as his wife and, later, ex-wife.

In the Season Eight finale in 2011, Larry accuses his neighbour, actor Michael J. Fox, of harassment.

Returning after a six-year hiatus, a fatwa, or religious edict, is issued on Larry for his impersonation of an Ayatollah religious leader in the Season Nine premiere, which aired on Oct 1.

•Curb Your Enthusiasm Season Nine airs on HBO (StarHub TV Channel 601), StarHub Go and HBO On Demand (StarHub TV Channel 602).

REBOOTS

DYNASTY

Original run: 1981 to 1989

This over-the-top soap opera about the filthy rich Carringtons starred the late John Forsythe as the oil magnate patriarch Blake and was memorable for the catfights between his new wife Krystle (Linda Evans) and ex-wife Alexis (Joan Collins) - not forgetting the towering shoulder pads. It was the No. 1 show in the US for its 1984 to 1985 season.

It ended its run in 1989 after nine seasons, with the fates of several major characters hanging in the balance, and was followed by a two-parter reunion in 1991.

The reboot premieres in the US today. Grant Show (on Melrose Place from 1992 to 1997) steps into the boots of Blake Carrington. Other tweaks include moving the setting from Denver to Atlanta and changing the Sammy Jo gold-digger character from a woman to a gay man. The creative team includes Josh Schwartz, who also created teen soaps The O.C. (2003 to 2007) and Gossip Girl (2007 to 2012).

•New episodes of Dynasty will be released on Netflix every Thursday starting tomorrow.

MIAMI VICE

Original run: 1984 to 1989

Crime-fighting detectives in spiffy, pastel-coloured get-ups and a cool soundtrack of pop and rock hits? It can only be Miami Vice. James "Sonny" Crockett (Don Johnson) and Ricardo "Rico" Tubbs (Philip Michael Thomas) were cops for the MTV generation and they dressed the part.

A potential reboot is reportedly being driven by movie star Vin Diesel and screenwriter-producer Chris Morgan, who have worked together on The Fast And The Furious movie franchise.

CHARMED

Original run: 1998 to 2006

The Halliwell sisters - Prue (Shannen Doherty), Piper (Holly Marie Combs) and Phoebe (Alyssa Milano) - are a trio of witches who use their "Power of Three" to protect the innocent from evil beings. After Prue dies at the end of the third season, their long-lost half-sister Paige Matthews (Rose McGowan) emerges. In the Season Eight finale in 2006, Piper has to travel back in time to undo the deaths of Phoebe and Paige.

A proposed CBS reboot in 2013/2014 did not get off the ground, but The CW television network announced in January that it is going ahead with a new version helmed by Jennie Snyder Urman, creator of Jane The Virgin (2014 to present), possibly for the 2018/2019 season.

Reacting to the news, McGowan tweeted a photograph of the original four Halliwell sisters with the word "irreplaceable".