Bass guitarists often use a technique of playing "riffs" against a chord progression. A riff is a pattern of notes (usually for one or two bars duration) that is repeated throughout a progression (or song). The following riff uses eighth notes and has two sharps, C# and F# .
In this riff both C notes are sharpened. This is because of the rule that a sharp (or flat), when placed before a note, affects the same note if it reoccurs in the remainder of that bar.
For ease of playing, use the 1st and 3rd fingers as indicated, and when changing from E to F# leave the first finger down in preparation for the next E note. This riff can be applied to the 12 Bar Blues in A. As discussed in the previous paragraph, be sure to observe the rule for playing sharps in each bar.
You have probably heard this riff style of playing before. Play some recordings (’50s Rock and Roll or Blues songs would be best) and listen to the bass guitarist.