But essentially, Charmed was created by Constance M. Burge who wanted the show to focus on magic and sisterly bonds as she based the main protagonists - 3 sisters; Prue (Shannen Doherty), Piper (Holly Marie Combs) and Phoebe (Alyssa Milano) on herself and her sisters.
Doherty (who passed away last month) was contracted first and was considered the main protagonist of the show, she would even go and direct as early as the 2nd season finale.
Doherty, who was known to be very passionate and voice her opinions, and who was best friends with Sarah Michelle Geller of Buffy probably saw the potential the show had and wanted the tone to be go more serious and dark.
The other executive producer, Bran Kern, wanted to focus on romantic interests and skimpy outfits - they butted heads a lot.
Over the course of the 3rd season (which would also be Doherty's last), tensions between her and Milano grew to a high, so much that the producers had to reportedly reduce to a minimum their characters' interactions despite them being sisters on screen.
Basically, Milano bamboozled Doherty and threatened to sue if Doherty wasn't fired (and it's safe to assume she was backed by Kern considering she was his favorite).
Doherty, despite directing the 3rd season's finale - All Hell Breaks Loose - one of the series' most intense episodes ever, was reportedly later fired over the phone.
This firing would eventually tarnish forever her career.
Rose Mcgowan would be brought into the scene to play the 4th long lost sister and does an extremely good job! However, Constance M Burge would also be bullied into being a simple consultant over her own show before departing completely by the end.
The show was envisioned with Doherty as the main selling point and Doherty, who was also best friends with Combs, encouraged her to audition too.
Even though Combs wasn't retained, Doherty went and said that if Combs wasn't on the show, she wouldn't be either forcing the producers to hire her.
So we can say Combs was loyal to Doherty. After her firing, Combs also wanted out but because of contract obligations, she was - by her own words - blackmailed into staying in the show with the producers claiming they would take away all her revenue be it from an acting gig or if she was in a faraway town working at a grocery store.
Enter season 5 and Combs and Milano become... PRODUCERS!
The show turns into a self-serving fest with the character of Phoebe becoming the best creature that ever lived - she is claimed to be the most beautiful, outstanding, gentle, kind blablabla, goes out with the richest boyfriends ever, all the villains are scared of her, she is super successful (over being a love columnist at a random newspaper - in the season opener we discover she literally has her face plastered over giant billboards it's crazy!).
Phoebe does the most selfish things ever and it's always the other person's (typically a man) fault. I kid you not, the characters actually do say it's the other's fault!
Rose Mcgowan, whose character remained the only likable one because her writing wasn't interfered by the 'producers', in the recent years revealed that every time the show would be renewed, she would cry because working with Milano was a nightmare.
Apparently, Doherty too cried when she knew the show was renewed and had to continue to work with Milano.
Piper on the other hand, I guess because Combs never wanted to be on the show by now, turns into a snarky character - a mother, now she forever stays at home frustrated that she got magical kids, commands everyone around, always annoyed at her husband and asks him to go do chores (basically in Combs and Kern's minds, a strong female figure is a condescending husband with boobs).
Past season 5, it's essentially the Phoe-ME era
By the last 2 seasons, every other person of the magical world has to beg them to help them because oh, they are so great (barf!)
At one point it even becomes ludicrous when they even argue with beings akin to primordial that they 'did a lot of good and they deserve to be given what they are asking for'.
In the last season, the Angel of Destiny has to literally kill Piper's husband because they have become so selfish and lazy in order to 'motivate' them into being heroes (sounds familiar?).
Because of Milano's ego, Phoebe turns from being the most wholesome character to the most horrible one.
Phoebe S1: 'Come on guys! We are witches! We have to help people! That's what we are!'
Phoebe S8: 'Oh honey... I have so much work... If we help people, demons are gonna come after us and we won't be able to help people... Sorry.'
One of Phoebe's opening lines in the later seasons is, I kid you not, 'I can't go out saving innocents because I have dates! LOTS AND LOTS OF DATES!'
If you are interested, check out Alison Preglers videos, they are great!