As/when/if the spirit moves you
Definition 1 of 1
Pro Tip 1/3Spontaneity
The phrase often suggests a spontaneous action, done without planning. 
I might join the game later, if the spirit moves me.
Pro Tip 2/3Personal Desire
It emphasizes acting based on personal desire or inclination, not external pressure. 
Feel free to contribute to the discussion as the spirit moves you.
Pro Tip 3/3Casual Invitation
Used in invitations to indicate the guest has the freedom to decide. 
Drop by for coffee anytime, when the spirit moves you.