This is what as known as "scope creep" -- simply put, when your scope of work is slowly expanded by your employer or customer incrementally, but without an increase in pay. In some cases, this is a conscious and deliberate attempt to add more value to their expenditure without adding additional cost. It's an unscrupulous and unfair business practice that is increasingly becoming the norm. Stop it, ya cheap bastards --- you know who you are!
I've worked in the corporate world -- I've seen it happen a lot there. But, I see it in music productions, too. In fact, I'm living it right now. I get hired to play the guitar. But, I also sing well. And I know how to run a band, as well as play other instruments, although I don't really advertise any of that, unless I'm specifically asked. It's hard to be in a production for any period of time without that sort of thing getting out, though. If you are also a singer, though, sometimes you open your mouth long enough for something musical to come out.
Try to avoid that at all costs.
Unless, of course, you want scope creep to...[ahem]...creep (sorry!) into your work. Now, I'm stuck singing backing vocals on half the show (for no additional pay, natch). Then it's "Oh hey, I figured you wouldn't mind singing lead on this song..."
[Me] "Um....ok..."
"Hey, that sounds great, here's 2 more for you to sing lead on..."
[Me] "Um....ok..."
More pay? Nope. Now, of course, I could say something to the effect of, "Ya know, you're really only paying me to play the guitar, ya cheap bastard!"....There's one problem with that, though: will they remember that the next time they hire a guitarist -- are you someone that provides more value to the client??? Maybe, maybe not. Are you willing to take the risk and lose a plum gig for your principles?
Principles, schminciples -- I gotta eat!
"Hey, since you know this material better than anyone on the gig, can you be Associate Musical Director?"...that means when anyone else in the cast has questions about the arrangements or the new key or whatever, they can come to me and ask for clarification, etc.
More pay?
Ohhhhhhhh, I think we both know the answer to that....