Quickie
Enter the scene as if you'll be there forever.
Never enter the stage with the idea to leave it immediately again. This is about actors entering a scene to deliver a single line, mostly aiming to throw in a quick joke as a waiter, messenger, or what have you.
Don't do that, since it's really just you on yourself having a little plan in your head. Moreover, always be prepared to be kept on stage by your partners, you don't know what's going to happen.