You hear all the time on these late night talk shows about the guest being so important and busy that they can only give the current host and show that they are talking to at the time so much time before they have to go to their next appointment. Whether they have to rehearse the show they are currently working on, rehearse a scene in a movie that is coming up, rehearse some new album that they are working on, negotiate their latest divorce settlement, meet their mistress, see their drug pusher, talk to their analyst, you know whatever it might be.
But apparently Steve Martin wasn’t important enough in late 1978 and had nothing else to do that day and could’ve given Johnny Carson all his time and done the entire show together. He could’ve filled in as the couch potato and given Ed McMahon the day off. He and Johnny could’ve made a cake together, he could’ve picked up Johnny’s laundry or met his girlfriend because Johnny needed to be with wife that night. Whatever Johnny needed Steve to do for him, looks like Steve had the time to help out and be a good personal assistant for Johnny, since he wasn’t important enough to have to leave early.