I've come in rather late on this one Philly, and reading through the many informative answers,
I will probably just be re-iterating many existing points, but here goes - as it certainly worked for me when
A) I did used to gig in a covers band - and B) When I 'started over' with the chord melody stuff :
Set yourself a virtual repertoire / gig list - as if any of the people on here had asked you to fill in on a gig, in 3-6 months time - and just sent a list . As it is your 'Virtual gig ' at this stage, use approachable - but currently challenging personal favourites.
1)
Initially, Assume your required 'duties' will be Rhythm guitar and 'fills' - and then set out to meticulously go one song at a time , pausing and going close focus on alternative chord fingerings if doing so does not collide with the general 'sound ' of the piece.
2) Learn any 'new' chording and or riffs/ fills that song alone needs you to know. That is to say, do not go at chord work, riff learning and such in a generalised way. Only download / read/ experiment with a new concept if it is the very next thing you need for that one song.
3) Ditto for next song. Start every practice session by working on the new material , and finish with a revision of the previous pieces, NOT the other way round. Apparently the brain 'etches' / re-enforces new paths better that way round.
4) As the song list grows, you will have added / researched / understood a great deal more than you thought you could at the outset ; and it will all have context and relevance , and be easier to remember - than if you just learned 'The big book of riffs' e.t.c.
5) At the end of your set list, you then imagine that you get an e:mail . telling you that you may also have to fill in, if the solo player breaks a string, or double up for sonic effect. This takes the weight off you getting it all note perfect in your mind, and instead gives you the fun task of simply getting better at whatever you can do by the time of the 'virtual' gig.
6) For learning the lead work / scale usage ( relevant to the underlying chord ) think only of the one phrase, riff, technique that the player is doing RIGHT NOW - not how hard the whole of the solo(s) might be to learn.
Over time, this will have naturally presented so many examples of ' apt scale over given chord' in a memorable and practical way - that you will not have to rely upon a general knowledge , but can instead, hear in your head how it actually worked.
Again, ( providing it is relevant to this part in this song ) - by all means find the same phrase somewhere else on the neck, or think of what you could ad-lib, if you had to play solo over that specific part of the chordal progression, without repeating the 'first' soloist . For example , Think of the second solo in Thin Lizzy's "Still In Love With You" - or Snowy White ( Pink Floyd 'The Wall' era ) - having to play a tasteful / relevant version of the solo from " Another Brick In The Wall - Part Two" live on stage - only a few seconds after Dave Gilmour had just played the official / recorded version .
So :
Set a target / structure of real songs
Impose a good natured time limit ( in months)
Focus only on the immediate song
Research / download / blag / practice / understand only what is needed for the immediate bar / phrase.
Rinse and repeat, but revise the existing material - at the end of each session of having learned new material ; even if that 'New material' is only a five note run - or new chord.
I hope that made at least some sense Philly - as it would be great to hear of you actually getting a chance to do the above for real, as a Guest player in the Band of one of the other forumites ! :)
Completely starting over , was the very best thing I ever did - and even now , I have recently decided to revise my previous Rock / Blues 'stuff' - as an adjunct to the main Chord melody project - as I realised that if anyone asked me to 'fill in' like the old days, I was losing the ability to do so .
Enjoy ! :D
