In the stage version of the Little Shop of Horrors musical, the plant didn't have a solo. I'm going to guess that was because they didn't think it could; that the puppeteer operating that monster of a puppet wouldn't be able to keep up with the lip sync of a long number. I'm assuming — and I could certainly be wrong about this — that the makers of the show thought it would look shoddy and make the plant look like more of a puppet.
When it came time to do the movie, that wasn't a problem. The plant puppet they build for the film was capable of some pretty convincing lip sync. (And by the way: A lot of folks thought it was animated or it was stop-motion or CGI or something. It wasn't. It was a full-size, real puppet operated by a large team of puppeteers. What they did though was to slow down its vocal track so the puppeteers could keep up with it and then after filming, they sped up the film to the proper speed. In scenes where the plant interacted with Rick Moranis, Mr. Moranis had to do his actions extra-slow so they'd look normal when the film was sped.)
Anyway, they also (I'm sure) wanted to add a new musical number or two to the film because when a stage musical is adapted for the movies, songs from the stage are not eligible for the Oscar for Best Song but newly-added tunes are. That's why in the film of Guys and Dolls, Adelaide sings "Pet Me Poppa" instead of "A Bushel and a Peck," and in the movie of The Music Man, Marian the Librarian sings "Being In Love" instead of "My White Knight." So the Little Shop crew added "Mean Green Mother From Outer Space" for the plant, Audrey II.
And it worked to the extent that that number did get nominated for Best Song. It was beaten out by "Take My Breath Away" from Top Gun but "Mean Green Mother" did get performed on the Oscarcast, albeit with a few lyrics sanitized for our protection. Here's a nice unsanitized rendition of it performed by Cavin Cornwall, who appears on a lot on stages and who, among his many voiceover gigs, is heard all over the last few Star Wars films…