Is there any reliable information as to the depth of Isaac Asimov's involvement in writing the novel Nightfall (based on Asimov's 1941 short story) with Robert Silverberg? Were Asimov and Silverberg truly writing or otherwise working closely together? Or is the novel primarily Silverberg's work with Asimov simply giving his approval?
To quote Isaac Asimov himself from the chapter entitled 'Robert Silverberg' in his autobiography, 'I, Asimov':
In 1988, Marty Greenberg had an idea. [...] why not find someone who would take my story "Nightfall," now forty-seven years old, keep the story essentially as written, but add a detailed beginning and a detailed ending to it. [..]
Marty said that we could always arrange for me to have full approval of the final novel and even make changes if I felt it necessary. [...]
[...] I explained to Bob [Silverberg] that I didn't want gratuitous sex, unnecessary violence, or vulgar language in the novel, and he agreed to that, indicating that he would be satisfied to let me have the final word on any matters under dispute. When I said "Delete!" it would be deleted, and when I said "Change!" it would be changed.
Eventually, I received the extended "Nightfall" manuscript from Bob. Despite everything, I had fearfully anticipated receiving something I couldn't endure [...]
I need have had no fears. Bob did a wonderful job and I could almost believe I had written the whole thing myself. He remained absolutely faithful to the original story and I had very little to argue with.
In short, the central third of the novel is essentially all Asimov's work: it's his original short story with only some very minor changes. The first third and the final third are all Silverberg - but within Asimov's guidelines, and with Asimov's approval/veto.
As for the other two Asimov/Silverberg collaborations, 'Child of Time' ('The Ugly Little Boy') and 'The Positronic Man' ('The Bicentennial Man'), we know that Asimov saw Silverberg's original outline for one of them:
Bob has already outlined his version of "The Ugly Little Boy." I have seen that outline and approve it heartily.
Asimov's autobiography ends in 1990, and he died in April 1992. 'The Child of Time' (based on 'The Ugly Little Boy') was published in 1991, so it's likely that Asimov saw (and approved) the final manuscript. 'The Positronic Man' was published in 1992; Asimov was sickening during 1991 and gave up writing about August 1991, but it's possible that he also saw the final manuscript of this book before it was published.
With a little of Asimov's help, Robert Silverberg has expanded an earlier short story that Asimov has written. That short story has ended when the night has arrived, that is, much earlier than the novel. Also, the planet has a different name. See the description for the novel in Jenkins’ Spoiler-Laden Guide to Isaac Asimov
haven't read it in a while, but it "feels" like an Asimov original. So unless Silverberg's writing style is extremely close to that of Asimov himself I'd warrant to guess that Asimov did more than just put his name on the cover.
Can't look up information in the book itself right now, as I'm not at home with my library and the Kindle version hasn't been released yet (so can't order that).