I was walking up Northumberland Street a couple of days ago, and heard a chap singing Elvis karaoke - very pleasantly. It made me reflect on the minimal necessary attributes for solo singing (attributes which this man possessed).
Firstly you must have a pleasing voice - otherwise there isn't any point in being a solo singer. This is a quality which many professional singers lack.
Secondly intonation: you need to be able to sing in tune (or close enough - many decent solo singers have imperfect intonation, but this cannot go too far.
(In my opinion Maria Callas went too far in the direction of being out of tune; but then she did not have a pleasing voice either.)
Thirdly phrasing. There are plenty of singers who sound nice and sing in tune but who cannot phrase musically - they are pretty worthless, really.
Musical phrasing is extremely important - and I believe it is something innate that cannot be taught (although of course people who lack adequate technical attributes cannot phrase due to their incompetence).
(There are plenty of famous professional singers, musical instrument players, and conductors who cannot phrase. Among conductors - for whom phrasing is pretty much their raison d'etre - notable non-phrasers include Roger Norrington and Trevor Pinnock.)
Attributes which are good to have, but not essential to be a solo singer per se (although necessary to be a classical singer) are vocal range, breath control, nimbleness and volume.
For actors, it is vital that they can remember lines with sufficient accuracy and in sufficient volume (i.e. not be compulsive paraphrase-ers - this diminishes the script and prevents other actors from acting).
They must be able to make themselves heard clearly.
And they must be able to act - which is the equivalent of phrasing.
There are plenty of professional actors who are unable to act - indeed, probably most professional actors cannot act. They get away with it (and may have successful careers) usually by possessing one of two other attributes: an impressive-sounding voice and/ or good looks. But strictly, these attributes - which vastly assist in a career as an actor - have nothing to do with acting.