I think playing music, in the strictest sense, is best approached as a craft not an art. In other words, something that can be practised, polished, has technique and to some extent ‘rules’ (which you at least have to know in order to be able to decide when to break them!) and is not primarily about expression or creation. This is why it’s possible and indeed necessary to practise whether you feel like it, sometimes up to it, or not, and in a sense why it is possible to ever play someone else’s music or perform the same piece twice.
But … writing music can be a different matter. And the gigging band world assumes people get involved in writing. There are very few bands where someone turns up knowing what they want everyone to do; normally they’ll have lyrics, tune, maybe a guitar part or a structure – sometimes not all of those depending on how collaboratively the band writes. In my usual melody / lead instrument role, I’ll rarely get more than chords (sometimes not those if a guitarist-writer has just found an exciting-sounding set of finger positions – it does happen!) and perhaps a couple of suggestions about places to come in and drop out. And in practice, I usually produce a mixture of fixed lines and improvisation.
This too involves quite a bit of craft rather than art from my point of view. Backing, rather than taking over, someone else’s song involves getting to some extent into their style, their aesthetic and their expression; even while people usually form bands to add something they wouldn’t be able to put in themselves, and so there’s also an element implicitly required of bringing my particular musical nature to the table. And if improvisation seems by its nature unrehearsable, that’s only because it’s practised in a different way. You may play differently every time, but getting used to where the music goes, how the group you’re playing with behave not just obviously musically but in subtle cues, maybe facial or postural, that can tell you a lot even semi-consciously about what they’re going to do or want you to do, and becoming essentially fluent takes more practice than playing a fixed part, even playing it in tight ensemble and sympathy with the rest of the sound.
And then there are the bands (to be honest, as many as not) where everyone is actively encouraged to write songs of their own. And for some reason this is where I tend to trip up. Because the craft-type stuff almost always offers ways of plugging the gap if you get stuck. They may be a bit cheap and dull, but as long as they’re not overused they get you off the hook. Things like inverted pedal points (basically, sitting on one note – probably the key note, maybe the fifth – for a long time), with or without octave shifts and rhythmic breaking up; working out the basic chords from a mixture of ear and watching fingers and just arpeggiating them or chugging on double-stops that cover two of the notes; simply dropping out for a section (provided it looks deliberate) works well for a top-line instrument; and I have a personal improvisation favourite of doing a little figure of the first three notes of the scale, descending, in something like semiquavers, repeated: so you get a three-over-four thing and also a movement rapid enough that if the harmony isn’t chromatic the ear will usually accept the line as fitting with any chord. You can keep that up almost for as long as you like, especially with changes in dynamic.
But you can’t exactly do this with writing from scratch. I mean I suppose you can nab and adapt standard chord progressions, and basic units like verse-bridge-chorus structures, four-bar phrases etc. are so basic to western popular music we don’t think twice about them. But if you find yourself going ‘I really need a second verse of lyrics’ or ‘I think this chorus should be twice the length but I don’t know what to write for the second half’, you’re somewhat restricted in your options other than sit down, play over what you’ve got, listen to stuff that’s similar to what you want it to sound like, and keep trying things. There’s no technical ploughing on way through, and most of the shortcuts sound really terrible (evident rhyming dictionary fifth verses, anyone?). Sometimes the perspiration is not a replacement for inspiration but a search and a waiting for it.