I hope this isn't too obvious, but it wasn't to me until I thought of it, so please be gentle in any replies...
When you buy a supermarket rotisserie/ready-to-eat chicken that has been wrapped in plastic, there is often a lot of "gunk" on it when it comes out of the bag. Do not be tempted to throw this away, or to rinse it all off.
Half or more of the gunk is darkish-brown. This is the meat juices (the chicken was still warm when it was packed). The other half is lighter in colour — that's the fat. Chicken fat is not much good for anything, but the juices are free gravy, way better than Bisto!
Open the packaging on a board or plastic mat and get as much of the gunk off/out of teh chicken as you can, either with a spatula or with your fingers inside the cavity after cutting the string that binds the legs together. Pour all of it into a tall glass. Leave if for a couple of hours at room temperature. The fat will rise to the top, and you can throw it away. Keep the brown jelly in the fridge — it'll be fine for several days. Sometimes there will be little pink bits of dark meat in there — keep that, it's all goodness.
DP and I will sometimes have half of the chicken in sandwiches for lunches, then one evening we pick the carcass, melt the jelly, and warm all the bits of chicken up in it. Throw in some peas that have had 4 minutes in the michaelwave, and maybe a couple of leftover boiled potatoes, and you have something not a million miles in taste from a roast chicken dinner in one pan and about 10 minutes!