But, honestly, looking for work as a musician, being able to improvise off of a lead sheet is a good skill to have. I know there's a lot of gigs out there where you just kind of flip through the 'Real Book' and take turns comping/soloing over whatever song you happen to turn the page to. If somebody put a piece of sheet music in front of you that you've never hear
 d or pla
 yed before, how would you put together a solo that sounds good/interesting to you? Can you sight read the melody, but instead of playing the melody you're reading, come up with a variation of that melody on the spot as you're reading it for the first time? That seems really difficult.
there are a couple of strategies. when you start to know a lot of songs, you start to recognize where a song is going and what it's doing, especially if it's a relatively 'normal' type of song. Look ahead, like to the whole line, and you should be able to get an idea of what's happening, at least if it's anything reasonably 'normal' going on. it should jump out at you. the more songs you know and the more experience you have, the easier this will be. But some songs might be more unexpected, experimental, and you might not be as sure where they're going. So if things start to feel uncertain, that's where you have to just start to zoom in slightly closer, and only look at 2 bars. worst case scenario, you JUST look at the one chord, and then just use the 'available notes' in a technical way. But usually you should be able to zoom out to AT LEAST 2 chords, or hopefully more. and the farther you zoom out, the better an idea you should get of what's going on. but let's say it's worst case and you really aren't sure where things are going and you are just looking at 1 chord, i still think 'scales' are not quite the right way to look at it. Just be aware of what the good notes usually are, and sort of rank them. The real good notes will vary depending on the context of the song obviously, and if you really know the song this will change everything, but as you said you are looking for tricks to use when you really don't know the context and all you have is a chord symbol staring you in the face. just sort of rank the notes. and there are all kinds of clues that will change things. so let's say the chord you're looking at is Fmaj7. well even still you can look at context. is the song in C, or in F? that will usually change the context. but on Fmaj7 your strongest notes are going to be E, A, and C. next probably G. If the chord is Cmin7, your strong notes are going to be Eb, then G, then Bb. again, context will always change this, but these are sort of the technical best options for when you really don't understand the context, for whatever reason (like if it's just a strange song doing a disorienting thing). for dominant 7th chords, there are a million zillion contexts for those chords, so you're really gonna have to know the song to figure out what's going on there, but your guaranteed available notes for example on G7 would be B and F. beyond that, anything else is kind of risking it, because 7th chords can be in all kinds of crazy contexts. sometimes specifics will be added. so let's say if it says G9 (which means G7 with a 9 on it) then you've got B, F, and A for sure. almost even favor A, because if the writer wrote G9, that must mean the A note is important. so just go for that note. if it's G7b9 then you'll be fine with B, F, and Ab. Zooming out a little bit more to a 3 chord sequence, things get clearer. since you mentioned the real book, I'll mention certain things that would occur there. if you're in Cmaj and you see D-7, G7, Cma7, you should really be looking at that whole figure altogether. basically what that figure means is "we're coming in for a normal landing on Cmaj." so just, i dunno, just get used to stuff you can play over that. don't think about the chords individually, think about coming for a landing on Cmaj at the designated time. your F note is going to be in the running for the entire time BEFORE the landing, and when the landing hits you will want to be on E, G, perhaps B, maybe D. prior to the landing you will want to be using notes like D, F, also G, A, etc.. Looking at the case of a minor key version of the same thing, the familiar pattern (again speaking in terms of the real book) would be something like Dmin7b5, G7b9, Cmin. again, this means "coming in for a landing on Cmin"... resolving to C minor is a little bit more interesting, and a bit more crazy things are usually possible. Ab note will feature prominently BEFORE the landing, and you will want to land on G, Eb, perhaps D if it's a Cmin9 specifically, etc. before the resolution, B, D, F, also Eb, Bb should all be fine. When a G7 resolves to Cmin in the key of Cmin, that is an especially strong case of "not all notes behave equally" -- this is really always true, and again that's why i don't like to use scales because scales implies that all notes are equal. and that's really not true. it's like characters in a movie. sure all the characters are there, but they all have different personalities, have different roles to play, and are featured with different levels of prominence. Again, in general when improvising, always zoom out as much as possible and try to gather a sense of where the landing points are, identify when it's getting to where it's going. usually you can tell pretty clearly by zooming out, and it gets easier the more experience you have. If you have a clear idea of where the thing is going by the end of the line, or by the beginning of the next line, you can really lean back and go wherever you want to go in the meantime, because you know very clearly when it has to get to where it has to go, and it will feel fine. it's like going down a staircase. you can do as many backflips as you want and you don't even have to touch any of the stairs as long as you land at the bottom of the stairs at the right time