Keeping meat juices to make gravy
It sounds like you are trying to maximize the amount of pan juices, sometimes referred to be the French word jus.

Any piece of meat is only going to express so much jus; if you have potatoes or other absorbent vegetables in the roasting pan, they are going to absorb it and it won't be available for another purpose such as gravy.

What you still will almost always have is fond, the brown roasty bits at the bottom of the pan.  Many cooks deglaze the fond with a small amount of stock or wine to create the basis for a sauce or gravy, as the fond has a very rich and complex flavor&mdash;and its flavor may actually be enhanced by aromatic vegetables in the pan.