Angel invited bands to submit demos for the theme music to the show. They asked bands to use "dark superhero ideas" and "cello-rock". For inspiration, Darling Violetta watched pivotal Angel-related episodes of Buffy the Vampire Slayer like "Passion", "Becoming, Part One", and "Becoming, Part Two", along with co-writer and music producer Holly Knight. Eventually, Joss Whedon accepted Holly Knight and Darling Violetta's interpretation of an Angel theme as that most suitable for the show.
The theme has a slower tempo than the Buffy theme, with heavier use of acoustic instruments such as cello and violin. In 2005, Holly Knight and the band composed an extended version of the Angel theme called "The Sanctuary Extended Remix", which featured on the soundtrack of the series Live Fast, Die Never.
Joss Whedon and Alexis Denisof sang their own lyrics to the theme song in the DVD commentaries to the episode "Spin the Bottle": "Angel is a vampire, who solves crimes (yes, he does) with his friends (with the gang). Wesley will never get a leather jacket cuz he will never be cool (aww!) no matter what he does (this episode sucked)."
Season 1 of Angel had the smallest main cast in comparison to the rest of the show. It stars David Boreanaz as Angel and Charisma Carpenter as Cordelia Chase. Glenn Quinn (Allen Francis Doyle) appears in the title credits for the first 10 episodes (although he left the show after the 9th episode) before being replaced by Alexis Denisof (Wesley Wyndam-Pryce) for the remainder of the season, and stays in the title credits for the remainder of the series.
Season 2 features Boreanaz, Carpenter, and Denisof again, with the addition of J. August Richards as Charles Gunn. The background for Alexis Denisof's colored border also changed from the inside of a building to drawings in a book. Both of these are consistent for the entire season.
Season 3 features the same cast from Season 2, with the addition of Amy Acker as Winifred Burkle. This is consistent for the entire season. J. August Richards' background color has become a slighter darker shade. The opening credit cast images are now presented in widescreen. This particular season displayed each the characters in a color of the rainbow:
- RED: Open/Creator Credits
- ORANGE: Charles Gunn/J. August Richards
- YELLOW: Wesley Wyndam-Price/Alexis Denisof
- GREEN: Cordelia Chase/Charisma Carpenter
- BLUE (Light): Angel/David Boreanaz
- PURPLE: Fred Burkle/Amy Acker
Season 4 features the same cast from the previous season, with two previous recurring characters added to the opening credits. Vincent Kartheiser (Connor) was added for the full season (his only season as a regular), and Andy Hallett (Lorne) was added in episode 14. Charisma Carpenter is still credited, even after Cordelia's death in Inside Out and will be until the end of the season. Also, Alexis Denisof is once again in the last spot in the credits, and is now credited with "and" in front of his name.
Season 5 features the largest amount of main cast changes. Charisma Carpenter leaves the show, with James Marsters (Spike) being added onto the main cast in her place. Also, starting with episode 17 ("Underneath"), Amy Acker's credit is changed to show her more prominently as portraying Illyria rather than Winifred, and Mercedes McNab (Harmony Kendall) is added to the title credits for the remainder of the season.
- Each Angel cast member had a signature colored border appearing when they were credited. Colors were sometimes shared by two cast members, though never at the same time — Glenn Quinn's yellow background was inherited by Alexis Denisof, Vincent Kartheiser's dark blue background was inherited by James Marsters and Charisma Carpenter's green background was inherited by Mercedes McNab.
- The only actors that are credited for all 5 seasons are David Boreanaz (Angel) and Alexis Denisof (Wesley Wyndam-Pryce), the latter appearing in all but the first 10 episodes.
- David Boreanaz is the only cast member to keep his background from the first season to the final season.
- Alexis Denisof is the only cast member to have two different backgrounds.
- Amy Acker is the only cast member to have two different characters credited, Fred (3–5) and Illyria (5).
- The complete cast of the series includes David Boreanaz (1–5), Charisma Carpenter (1–4), Glenn Quinn (1), J. August Richards (2–5), Amy Acker (3–5), Vincent Kartheiser (4), Andy Hallett (4–5), James Marsters (5), Mercedes McNab (5), and Alexis Denisof (1–5).