Constance M Burge's original concept for the series was just about three roommates who become witches. The network didn't think that witchcraft was a bankable enough concept, and so the main characters were changed to sisters in the hopes that the family dynamic would draw in viewers.
Shannen Doherty initially passed on the pilot script when it was sent to her. A friend convinced her it was worth a second look and "made me watch Practical Magic", and Shannen signed on. The friend was Holly Marie Combs, who the network initially didn't want because she was an unknown. Feeling Holly was perfect for the role of Piper, Shannen threatened to quit unless they cast her. In that scenario, another actress was Aaron Spelling's first choice for Phoebe, but had to be dropped when the network told him he could only keep two of his preferred actresses. Lori Rom was then cast as Phoebe, but dropped out after the unaired pilot.
Additionally, Aaron Spelling wanted Shannen Doherty to play Piper and Holly Marie Combs to play Phoebe (she is actually the youngest of the three initial leads). Holly felt more drawn to Piper, and Shannen to Prue, and they actually had to show him their driving licenses to prove they were old enough to play those roles.
The opening was going to place Alyssa Milano's And Starring credit at the end, after Dorian Gregory (Darryl). They then decided that the three sisters would be billed before the supporting cast, and this tradition continued when Holly Marie Combs was given the And Starring from Season 4 onward.
Rex was intended to be American, and Hannah would be British, but it ended up switched around.
Jennifer Rhodes says that she was initially only contracted to play Penny Halliwell for two episodes, "Is There a Woogy in the House?" and "That 70s Episode". She ended up appearing in sixteen across the series, and being the only actor to appear as a guest star in all eight seasons.
Melinda Warren was intended to make more appearances, but Tyler Layton just wasn't available.
Misha Collins, who appeared in the episode "They're Everywhere", later revealed that his character was intended to be a recurring love interest for Phoebe. He didn't appear again because "they thought I was terrible" (it was one of his first TV gigs). He and Phoebe also filmed a kiss scene that was very obviously cut from the episode.
Julian McMahon was contracted for a seven-episode arc as Cole, presumably writing him out after "Sleuthing With the Enemy". Brad Kern however was so impressed with him that he was kept around through Season 5.
Tiffani Thiessen (who had replaced Shannen on Beverly Hills, 90210) and Jennifer Love Hewitt were approached to become The Other Darrin for Prue. After they both declined, producers decided to create a new character. Rose McGowan however was always the first choice for Paige.
A rumor was that Brian Krause wanted to leave after Season 5, citing burnout and the character subsequently becoming an Elder in the finale. Other sources say he wanted a pay rise for Season 6, and the plan was to write him out as a regular and have him only make recurring reappearances as a spirit guide.
Jesse Woodrow was meant to play the Prince in "A Knight To Remember" as well, implying that the prince was Glen's past life. He refused.
After Holly Marie Combs and Alyssa Milano were made Producers for season 5, Rose McGowan was offered the position too. She declined, saying that the only thing that changed was her name appearing twice in the credits. Although since there was no on-set producer from Season 5 onward, Alyssa and Holly filled those roles themselves.
The original storyline for Season 5 would have had Paige and Cole start a relationship. In the premiere when Paige casts the spell on Cole, he would have kissed her passionately under the influence. Both actors disliked the idea and it was dropped. Holly Marie Combs also said that Cole "left before his time", meaning that his Face–Heel Turn was going to be more gradual, than the abrupt shift midway through Season 5.
Greg Vaughan was asked to come back for guest appearances in the last couple of seasons and he was happy to, but he was on another show at the time, and the network just wouldn't allow him to, even when they were being shot on the same lot. This slot was then given to Piper's fireman boyfriend Greg from Season 6, who appears in "The Last Temptation of Christy".
Chris was originally conceived of as being future Wyatt when he appeared in the Season 5 finale. However, the showrunners changed their minds and decided he would be Piper and Leo's second son, because it was less obvious, before Season 6 was filmed. Contrary to urban myth, the change was not made because Holly Marie Combs got pregnant; her pregnancy just caused Chris to be born earlier than planned and made him look dumb for nearly preventing his own birth. He was also never going to be Phoebe and Cole's son as another rumor would have it.
In late 2013, soon after news of the reboot series was announced, Alyssa Milano sent out a tweet that floated the idea to her costars of doing a Charmed reunion movie. Shannen Doherty responded in the affirmative, also retweeting a fan suggesting funding the project through Kickstarter. After the success of the Veronica Mars movie through Kickstarter, the possibility is there. Fan press for a potential reunion movie was still strong after the tepidly received reboot was cancelled, but Alyssa Milano admitted it was unlikely to happen after her and Rose McGowan's very public falling out, as well as Shannen Doherty's health issues.
Shannen Doherty was in talks a few times about the use of her image on the show and making possible guest appearances as Prue.
For the episode "Cat House", which functions as a Clip Show, she was asked about archive footage being used of her. For whatever reason, they couldn't come to an agreement, so the only flashbacks of Prue are her on the back of the motorcycle in "Just Harried", where she was played by a stuntwoman, and her as a dog in "Look Who's Barking".
She was asked to come Back for the Finale but, depending on the source, either turned it down because she felt it wasn't true to the character or else wasn't asked at all because she was too expensive and/or to avoid making Prue the Spotlight-Stealing Squad. The former case could also be for the Season 7 finale.
She was approached about having her likeness made into an action figure along with the other three sisters, but she declined.
