In the dream I was performing in a stage revival of Star Trek: Deep Space Nine, and three of us were rehearsing. I'd been doing fine when I had the words in front of me, reading the script, but now we were on the stage and I had no idea what to say.
The other two actors clearly had veered off the script when I missed my cue and were looking at me askance, waiting for me to pick up – improvising until the time that I got back on track.
Finally they realized how lost I was, and they stopped the rehearsal and looked at me in disappointment: "You don't know your lines?"
There are many takeaways here – know your lines, for one thing. But just knowing your lines gives you art akin to a factory product – more important to understand your role well enough to be able to improvise when necessary, as my fellow actors were doing.
If the world's a stage and all of us just players, there is a place for someone who does not follow a script, who can follow the needs of the moment. There is a place for someone who can be relied upon to do the right thing – to know what the right thing is and to get done what needs to be done.
Knowing lines is a first step toward being able to improvise. The soaring guitar solo isn't possible if you don't know the chord structure or the melody.