Probably a dumb question, but here it goes: when a band writes a song, who writes the music for the individual instruments? Is it usually done by one person in the band, or does each member write the part for their instrument?
As an example, when I’m listening to a Led Zeppelin song, who wrote the bass line? Did Jimmy Page write the bass line, plop in front of John Paul Jones, and say, “Play this”? Or did John Paul Jones write the bass line?
It depends on the band and on how they write songs.
In the case you give us in the OP, Jimmy Page wrote the song. Whether or not JPJ writes his own bass part, JP wrote the song.
For a lot of songs, yes, the person who wrote the song will delineate the basic parts of all instruments and then those parts are played and/or embellished by particular musicians. I know that’s how I do it.
Most commonly in rock music, the songwriter writes the tune and the lyrics and then has the rest of the band members figure out in collaboration with him what would sound good with it.
A whole bunch of The Who’s stuff started off as demos fully written and played by Pete. He played the demos for the band and they took off from there.
This is definitely not a dumb question; it’s a very insightful question that a lot of people don’t think about. As has been stated, in a rock band situation, yes, usually the songwriter will come in and say, “Hey, listen to this tune”. The other musicians will gradually tinker around and come up with something that fits the tune, and of course the songwriter and/or producer might say, “Oh, try THIS”, or “Nah, that doesn’t work”.
However, if the band wants to add an orchestra (say, the strings on “Tiny Dancer” or something), someone will be brought in to do that. An orchestrator, to be precise (Paul Buckmaster, in this case). You can’t just say to an orchestra “Play what you guys think fits the song”, obviously, because they’re only coming in for one day, and let’s face it, violists aren’t exactly known for their improvisation skills. Not to mention they wouldn’t all be together, blah blah blah. So Buckmaster wrote out every exact note each instrument should play (the violins, violas, cellos, etc.).
So it’s really a mix of the two answers, depending on the orchestration of the song and the dynamic of the band members (i.e. lead singer/songwriters who have a big ego might not want as much input from the other band members, whereas a start-up band might be very collaborative). Interesting stuff!
As said above, it will vary from band to band, but, typically in my experience in rock bands, it’s not micromanaged down to the instrumental part level by the songwriter(s). In bands that are driven by a strong songwriting personality with a clear vision, typically what I find is that the main melodic and rhythmic hooks are outlined (sometimes in the form of an orchestrated demo), but the details are filled in by the players. But the principal songwriter may steer those players stylistically to match their vision, like “play something droney-Stereolabby here” or “I like that, but give me more movement in the bass, more walk or something” or “Gimme some kind of four-to-the-floor disco stomp in the chorus on the drums.” That sort of thing. Basically, a directorial type of approach. But it’s all over the map. I’ve been in bands that have been more collaborative and bands where there is a clearer vision, but I’ve personally not seen a case where the songwriter wrote each of the parts themselves.
Although, I should add, there are bands that are more micromanaged–like Smashing Pumpkins, where I believe Billy pretty much had all the parts in mind. So, like I said, it’s all over the place. But, in my experience, the details are typically left to the instrumentalist.