Hold on tightly, let go lightly.
Very Heather Urquharty of course to present the "kill your darlings" principle in a little rhyme. :-)
This is a very important idea, especially in musical scenes and joint songs, but also in general improv.
Stick to your impulses, make your intentions very clear, be overly specific. But at the same time, let go easily if things aren't getting picked up. Gladly accept when things take an unexpected turn. Be happy and generous even when you don't feel your partner is following you.
Your impulses are only there to inspire your partner. Apart from that, it's not about you. Leave your ego out.