Q: How IS a psychic sideman like a unicorn?
A: Neither one is real.
So why do so many musicians think they're psychic?
Players love to play, so when you're sitting in or subbing with an artist you don't ordinarily play with it's easy for us to get carried away. But I'm sometimes amazed by how some players act like they already know the song when they don't. Guitar players get a bad rap for this, but I've seen and worked with quite a few bass and keyboard players who are just as guilty. When you're playing a song for the first time, YOU DON'T KNOW WHERE IT'S GOING! You might have a good idea based on the artist's vocabulary and style, but even then it's impossible to anticipate every possible change.
So why do some people play MORE notes when they don't know the song? If you're playing more than listening, you're not serving the song and you're not doing your job. This is good advice no matter what, but if you're sitting in and don't know the music it's just disrespectful.
Not trying to call anyone out here. But I see it often enough that it's something that I believe needs to be said. Writers rounds are one of the most common settings for this to happen. At least listen to a verse and chorus before you start to jump in! If you understand song structure then you should have a decent sense of when a change might happen, and if you don't know what it'll be, back off and wait until you hear it. It's like you're approaching an intersection....common sense suggests that you would stop and look both ways until you know what's coming. Listen and react...you'll probably play something better and make the artist much happier. Also, a good musician won't play fills in the middle of a vocal phrase, or play so loudly they can't hear the song they're supposed to be supporting.
This is all good common sense, really, and it makes you a better sideman. Open ears and the ability to react musically are most important skills for a supporting musician, applying those skills makes the experience that much more gratifying.