When Snape climbs the steps up to the Astronomy Tower in *Half-Blood Prince* (The Lightning-Struck Tower chapter):

> 
> But somebody else had spoken Snape's name, quite softly. "Severus..."
>  The sound frightened Harry beyond anything he had experienced all
>  evening, Dumbledore was pleading.
> 
> 
> 

And then

> 
> “Severus . . . please . . .”
> 
> 
> 

Was this to keep up the illusion, or was there unfinished business, the plan was not ready that night? Did Dumbledore not want Harry to see?