Good thread, Rick.
Great advice, Art about the toilet paper, it's absolutely true and valuable.
When accompaning a singer or in a jam session people take turns with the 'lead' vocal or instrumental, don't play over the vocals. Typically melody or intrumental lines are not played over top of the vocal. Fit yourself around the phrasing of the song. Comp (that is play cords or use a comping lick, eg, boogie woogie) on your instrument during the vocals, and when you're up to it play fills between the phrasing of the vocal. Learning to be a "side person" will open doors for you, it teaches you to listen to the other performers and sync.