Our ‘go-to’ stroke for single lines is alternating the index with the middle, there are exceptions of course.
Alternating index and middle needs to be built up slowly, initially by planting (placing the finger on the string before it is sounded). Once this has been mastered, it should only be used for staccato playing as, of course, the notes are cut short resulting in silences between the notes, the opposite of smooth playing.
We need to practise the stroke so much that it is our automatic ‘natural’ stroke and so that we have to think about using anything else.
'Anything else' would be perhaps repeating the use of a finger, i.e. when a single note is repeated and exactly the same tone and weight is needed - a technique I use a lot, for Lobos’ 'Prelude Number 3' for example or 'Marcello's Adagio in Dm'.
Index and middle legato playing will discussed soon.