There are many things to consider when collaborating with others. I find these three to be very important in my own endeavors of collaboration.
Doing one's homework is important at all times, but even more important as we collaborate with others. There is nothing worse than being the "weaker partner." I'm certainly not talking about those times we have the privilege of being the "weaker partner" when we get to work with someone who is superior to us and will challenge us to new levels of expertise. I'm talking about those times we become the "weaker partner" because we're not prepared.
I work with a wide variety of musicians from high school students in a Solo & Ensemble environment to college music majors preparing a senior recital or auditioning for grad school to professional situations with other well-trained colleagues who can challenge me to new heights. No matter which situation I'm in, I never want to come in to the practice prepared. I know several great musicians who love to "wing it." Most of the time, this works well for them. In fact people rave about how good they are at sight reading or the fact that they wouldn't have time to look at the music ahead of time anyway--they're so good, they don't need to. I could personally play that role. I can sightread most anything and can "fake" my way through an audition. But, I don't want to put myself in that position and don't feel it is what I should ever do to someone I'm collaborating with. Be prepared. Mark up your score. Highlight the key signature or accidental you just might miss. Get to know the road map of the chart. It makes you a better collaborator and I've yet to find anyone who looks down at me because I'm prepared as opposed to "winging it."
Adapt to Your Partner
What instrument(s) are you playing with? What is their dynamic range? How can you enhance and play as an ensemble? What types of articulations are they able to do and how does this affect your interpretation of articulations? These are just a few of the considerations when collaborating. As a classical accompanist, I need to think about style and what I'm trying to create. Often times, an instrumental accompaniment is a reduction of an orchestra score. Some of these are written well for a pianist to play, but others are clumsy and need some adaptation to sound stylistically correct. Consider the music you are playing. When I play jazz, I certainly make some major adaptations to my playing according to who I'm playing with. If I'm playing jazz with a solo instrument, my playing incorporates the nuances of the whole rhythm section, but as we add players, I play less. I certainly don't want to add a lot of bass notes when I have a good bass player covering that area. It only messes things up. I think of harmonic ranges. Am I playing the same notes the guitarist is? Is there something I could do so we weren't all "stepping on each other?"
Listen and Learn
Every time I collaborate, I learn something. I might learn something about myself or I learn something about the instrument or voice I'm working with. Usually this comes from simply listening. This helps me grow and become a much better partner in collaboration.