Was it an independant film?

I thought TriStar Pictures was involved? Maybe they just distributed it. In any case, Superman The Movie was financed by the Salkinds outside of a studio as well, but they ran out of money and couldn't finish Superman II which was being made simultaneously, which is what led to the time reverse ending being plopped into Superman The Movie's climax at the last minute. They tried to keep WB's hand out of the pot, though I think they may have needed a little bit of funding from them at the end. I'll have to research that last point a bit, as I am not sure. In any case, maybe they did the same thing with Supergirl and financed the film themselves?

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Whose idea was it for the word "Lisp" to have an "S" in it?

yes supergirl was mostly financed by the salkinds, it took ages to be released because the original distributor backed out so they had to find someone else to distribute the film BUT in turn the new distributor wanted the runtime cut

this is why in the US dvds there is a directors cut with some of the footage back

BUT apparently there is still over 30 mins of extra footage in the vaults where they found all the superman 2 footage