Outlived Its Creator: Supergirl was created by Otto Binder (1911-1974) and Al Plastino (1921-2013). They contributed stories from 1959 to 1968. Other writers and artists continued writing Supergirl stories afterwards, and their deaths had no effect on the character or her popularity.
Running the Asylum: Some fans have speculated Kara Zor-El was brought back because several of the primary architects of the Modern DCU are fans of the Silver and Bronze Ages. Whatever the case may be, Dan Didio isn't really a Silver Age fan, and he okayed her return because he felt that Supergirl's backstory had become too confusing -four different Supergirls with increasingly convoluted origins since the death of the original character in 1985- and a simplified version was needed.
Writer Nick Spencer had big plans for Supergirl before he left partway through his first issue. He was planning on setting Kara up as a leader like her cousin Kal and would have led to the creation of a new Young Justice. The story plans go that the villain would have been the Luthor-Brainiac clone and, to deal with it, Kara would gather a team comprised of Static, Blue Beetle, the Damian Wayne Robin, the Stephanie Brown Batgirl, Miss Martian and the Iris West Impulse (with hopes to include Aqualad). The pinnacle of Kara's evolution would have had Iris running away in a panic and Kara stopping her, convincing her to keep going. Most of the team, minus Kara, would have ended up getting captured and, with advice from Superman, Batman and Wonder Woman, would have made a second team to rescue them. The story would have concluded with the dance party of the Flyover app's premiere, with one scene showing Iris dragging Damian out onto the dance floor. The Jaime/Damian/M'gann team (but none of the others) made it into James Peaty's run, but the details of the storyline were completely altered, along with Alex's true identity ( Kryptonian-Dubbilex hybrid clone).
Anj: You sowed the seeds of future plotlines which may never come to fruition. Can you tell us what you were planning with Blaze? Siobhan? Michael? Shay Veritas? TONY: I was going to have Blaze create a trio of henchgirls called the Furies. Remember Siobhans roommate and her two friends? They were all orphaned in superhero battles and had sworn themselves to finding a way to make super-people pay. They were going to beat Supergirl and take her back to Blazes home dimension. Hilarity would ensue. I wanted to get Siobhan in there more, to make her and Kara a team. I had one issue where the newly Red Lanterned Kara fights Silver Banshee, and I contrasted their fight with flashbacks of Kara and Siobhan just having fun roommate moments. That remains one of my favorite things from the run: just letting her have a friend. Michael returns in my final issue, and well see if they do anything with him after that. If not, it was enough that Kara met a guy who had every reason to hate life and yet retained a positive outlook. He was in many ways a good role model for her, and he didnt want to trick or use her, which is nice for a change. And Shay I wanted to keep playing sort of mysterious. Is she really a friend to Supergirl, or does she view her as another science project? Shays still a bit of a cypher to me, which is okay if shes played for mystery. But I liked doing scenes of her staff at the Block talking about what its like being stuck there, having to wear her face, alternating between admiration and resentment. It gave the whole Block set-up a little complexity.
Geoff Johns had wanted to include Supergirl in the "One Year Later" Teen Titans roster, but since she was going a Darker and Edgier route at the time and Johns wanted a " naïve, fun alien chick", Miss Martian was created as a substitute.
Blooper: In one of the scenes with Selena and the protesters, a large puddle of water suddenly appears. This is because in an extended version of the scene, Selena turned one woman into an ice statue and shattered it. The sequence was cut but the water remains.
Dawson Casting: The casting of Kara isn't too bad by the usual standards of this trope; Helen Slater was 20 at the time, playing a high school age girl. But Lucy Lane? Actress Maureen Teefy was 31 at the time the movie was made! And, well... let's hope Jimmy Olsen (played by the 27 year old Marc McClure) is an example, as the alternative interpretation is more than a little creepy.
Executive Meddling: Tri-Star Pictures cut the film's runtime down from 138 minutes to a mere 105, leaving glaring continuity errors, and the film subsequently made less sense. A Director's Cut was released on DVD in 2000, along with the U.S. debut of the "International Cut" which runs 20 minutes longer than the American version.
Franchise Killer: The film's failure not only prevented Supergirl from becoming a film franchise like Superman, but was also partly responsible for getting the character killed off in the comics as well. It also led to the Salkinds selling the Superman film rights to Cannon, leading to the infamous Superman IV: The Quest for Peace.
The movie was originally going to use a brand new costume design for Supergirl, and there are even test shots of Helen Slater wearing it. The producers changed their minds at the last minute, and decided to go with the classic Supergirl look instead. However, the comics had already adopted the new look to better line up with the movie, and Supergirl ended up wearing it until her death in Crisis on Infinite Earths.
An early draft of the script had Bianca as Selena's advisor and mentor. With her help, Selena launched a coup to take over ruling the Circle from Nigel, then betrayed Bianca and left her mad after being corrupted by her newfound power. Ethan and Supergirl would then free Selena, initiating a HeelFace Turn. Selena was Easily Forgiven and left to go to Argo City with Kara.
Brooke Shields was so close to getting the role of Supergirl but was ultimately overruled at the urging of co-producer Ilya Salkind, who wanted an unknown. It was a decision Ilya later regretted. Melanie Griffith was also considered.
In Superman: The Animated SeriesPaul Dini and Bruce Timm wanted to use the original Supergirl, Kara Zor-El, instead of the then-current Supergirl (Matrix/Linda Danvers in the comics), but they were denied by DC Comics, due to the then-current "Superman is the only survivor of Krypton" edict.
There was talk of a Justice League-style cartoon way back in the late 90's. The team would have included Supergirl.
Legion Of Superheroes: According to James Tucker in an interview with World's Finest Online, had the show continued with a third season, Supergirl would be introduced.
Young Justice: Christopher Jones has stated that had the show been renewed for a third season, Supergirl would have appeared. Given that the show has been Un-Cancelled, it could be possible for her to make an appearance.