All good advice (as usual) from Kid. If the music editor did a good job and cut at downbeats, that makes your life easier. The problems come in when they say something like, “We want a hard out now instead of a fade out,” and that hard out doesn’t fall in a good musical place (BTW, I meant musical phrase before, not necessarily a vocal part, which can still be a problem if you’re in the middle of a melody).
You can tackle these in many ways: time stretch the music to fit, (but then other points might not hit where you want–and audio quality might suffer), add/subtract bars (keeping in mind that you may have to try to make a 2/4 or 5/4 measure in a 4/4 piece sound like it was meant to be there), use the process tempo function (if there are tempo changes) etc. I’ve even music editors slice up every beat of a song and compress the timing. Yikes!
Luckily, I find that at least 50% of the time, “dumb luck” plays a part and you can do something simple like add a measure or take one out and it works out well.