Some of the rumours (not confirmed folks) from googling the net, courtesy of the Charisma Carpenter discussion forums:
1.) Christian Kane originally auditioned for the role of Riley on BTVS. He didn't get that part, but producers like him so much he was offered the role of Lindsey on ATS.
2.) David Fury had originally wrote a much darker script for Season One Episode Two. The script had Kate taking drugs and being a prostitute, while Angel licked blood off of a murder victim. It was later changed to "Lonely Hearts."
3.) Doyle's death was planned in advance, and Glenn Quinn knew the role would be short term going in.
4.) Joss Whedon's portrayal of Numfar during the Pylean saga was kept a big secret initially. The only ones that knew where David Greenwalt, Tim Minear, David Boreanaz, and a few key makeup people.
5.) Waiting in the Wings was created after Amy Acker mentioned to Joss Whedon that she used to be a ballerina. There was even an extra ballet scene filmed in which Wesley and Fred replaced the leads.
6.) The character of Gwen Raiden was originally supposed to debut in "Deep Down."
7.) The episode "Players" contains several hidden dirty jokes.
8.) The producers had originally planned for Cordelia to wake up and kill Jasmine in the next to last episode of season four, but plans where changed after Charisma suffered complications during childbirth, and her doctor wouldn't release her to do any physicality.
9.) In "Awakening," the reason Angel said "Buffy" was because he had said it the two previous times he switched from Angel to Angelus, and Angelus to Angel on BTVS.
10.) The C/A ship was killed off because it was unpopular with the fans.
11.) Originally there was not going to be a Jasmine character in Season Four. Cordelia was just going to be evil.
12.) The original ending for season four was a big battle between Angel and Evil Cordy.
13.) Though they didn't come right out and say it, the producers implied to Charisma that she should get an abortion.
14.) Andy Hallett was added to the credits with nine episodes left in Season Four, because they thought the show might be cancelled.
15.) Gunn's urinating on Angel's desk in "Life of the Party" is based off an actual incident that involved crew members on BTVS.
16.) Michelle Trachtenberg had originally agreed to reprise her role as Dawn for "The Girl In Question," but had to cancel after the shooting schedule on her movie was changed.
17.) The final six Angel episodes contain several hidden potshots at WB executives.
18.) Sarah Michelle Gellar was asked to appear as Buffy Summers in Angel's 100th episode, but had to cancel due to a family death.
19.) Charisma Carpenter turned down Mutant Enemy's inital offer to appear in "You're Welcome."
20.) The original script for season five episode 17, featured Harmony getting her soul back, and the return of Lilah Morgan.
21.) Vincent Kartheiser (Connor) had signed a multiyear deal for Angel, but was fired after bringing up concerns over how his character was being handled.
22.) David Boreanaz and Christian Kane were very uneasy over the Lindsey posing as Doyle storyline in Season Five.
23.) Christian Kane was brought back to Angel as a favor to David Boreanaz.
24.) Mercedes McNab was added to the opening credits for the final six episodes, because the show was being cancelled.
25.) Nicholas Brendon had originally offered to go over Angel after BTVS ended. However, Joss Whedon turned him down, because he "wouldn't fit in."
26.) Had there been a season six, Seth Green was supposed to return as Oz to help Nina deal with being a werewolf.
27.) Navi Rawat was scheduled to return for more episodes as Dana the psycho slayer.
28.) There were two plans regarding Fred/Illyria. One was to have Willow seperate them, while the other was for Illyria to start taking on more of Fred's characteristics.
29.) Had Angel gone to a season six, Tom Lenk (Andrew) was going to become a regular cast member.
30.) The contracts for Alexis Denisof (Wesley), J. August Richards (Gunn), and Andy Hallett (Lorne) had expired, and they were not going to be brought back for season six.
These were supposed to have been confirmed as being true
1 - Also Christian and David are friends, so that couldn't have hurt with him getting the role of Lindsey.
2 - I have the original script for "Lonely Hearts" on my other puter, I'll post it soon, if anyone's interested
3 - The rumours about Doyle being killed off because Glenn Quinn and CC didn't get along weren't true, it was always planned for Doyle to die early on in the season.
5 - Amy revealed this in an interview a couple of years ago.
10 - The A/C ship was stopped for many reasons, but one major one was that fans didn't like it.
11 - Plausible, seeing as no-one knew CC was gonna get pregnant.
16 - Michelle had agreed to do the episode, but was busy filming. I believe Joss spoke about it in an interview.
17 - "Jump the Shark" anyone?!
24 - From what I understand MMcN was added to the credits due to the shows cancellation and they producers thought that after 8 years in the BtVS/Angel universe it might be nice for her to put it as a 'regular' credit on her resume.
25 - Joss and Nic have both spoken about this in interviews. Nic was keen and so was Joss, but they couldn't find a place for him.
26 - Not sure about the validity of this one, though I have heard a lot about it, so it's more likely than a lot of other crap doing the rounds.
28 - The plan regarding Fred/Illyria was to have Illyria take on more of Fred's personality as she had slowly begun in the final episodes. This was revealed by one of the writers, but I can't remember who.
34 - True
