While Christopher is not wrong, this is the part where I have to say that, in fact, I have, on several tie-in projects, been handed an outline of what to write. In most of those cases, it's a pretty simple outline rather than a detailed one and I have to embellish and add to it (my StarCraft novel Nova, for example; my Heroes Reborn novella for another).
To keep this to Star Trek....
Diplomatic Implausibility: "Hey, Keith, how'd you like to write Worf's first ambassadorial mission?" Plot then worked out in collaboration with John Ordover.
Demons of Air and Darkness: Plot worked out between me and Marco Palmieri, who had a very clear vision for how he wanted the DS9 post-finale novels to go.
The Brave and the Bold: I pitched this is a multibook "starship team-up" series, John told me to slow my roll and make it a two-book series like Susan Wright's The Badlands. The debut of Enterprise forced a last-minute change to the prologue....
The Art of the Impossible: This was pretty much entirely my story, as I came to Marco with the notion of explaining the Betreka Nebula Incident.
A Good Day to Die and Honor Bound: Very much a collaborative effort between me and John. For one thing, the notion of turning the Order of the Bat'leth into the Knights Templar was his idea....
Enemy Territory: This was all mine.
A Time for War, a Time for Peace: This whole project was originally conceived over dinner by me, John, Robert Greenberger, John Vornholt, and Carol Greenburg. It went through a lot of changes, though I insisted from jump that I do the finale and that I get to write the story of how and why Worf was back in Starfleet in Nemesis.
Ferenginar: Satisfaction is Not Guaranteed: I don't remember if Marco came to me with the idea of doing Ferenginar or if I begged him to do it so I could for once do something 180 degrees from the Klingons; we worked out the plot together, though this one was mostlyme.
Articles of the Federation: "Hey, Keith, how'd you like to write a Trek version of The West Wing?" That was John who asked the question, but it was Marco who took over the project and made it work; most of the mini-plots were mine, but several were Marco's, notably the Trinni/ek state dinner.
The Mirror-Scaled Serpent: "Hey, Keith, how'd you like to write the Voyager MU novel?" Having said that, having the bulk of it take place in Ardana and having the cloud city crash was Marco's idea.
Q & A: "Hey, Keith, how'd you like to write one of the 20th anniversary TNG novels?" This was entirely mine, and a story I'd been wanting to tell for years.
A Burning House: This was mostly mine, with significant kibbitzing from Marco.
A Gutted World: This was also mostly mine, with significant kibbitzing from Marco.
A Singular Destiny: "Hey, Keith, how'd you like to write the first followup to Dave blowing everything up?" Marco wanted to have there be one novel that showed the broader galactic consequences of Destiny and help set up The Typhon Pact; this one was very much a collaborative process.