When in Doubt, Hold a Note
In jam situations sometimes songs are called quickly and the person counting in looks to you to solo in the intro. You may not know where in the form the song is starting, nor the feel of the song (even if they call a "Shuffle," there are many types of shuffles). You could just start blowin' riffs, but an even better approach is to play a long-held note (with vibrato would be a good idea) for about two bars... and then continue playing.
This long-held note will sound good, at the same time allowing you to listen to the feel of the song and appropriately pick your notes/licks to come. As you hold that note you'll also hear where you are in the form.
P.S., the "When in doubt, shake" tip still applies. The 4 draw shake, with the 4 draw as its central pitch, matches both the I7 Chord (5th) and the V7 Chord (root), which are the two common chord options for openings.
More on openings tomorrow.