Petitioners

According to the 3rd edition rules, outsiders (including petitioners) need to breathe, but they don't need to eat or sleep. In second edition, though, some outsiders did need to eat and sleep.

If you decide that petitioners need to eat, they don't necessarily need to eat physical food. They might subsist on the glory of their deity, or "feed" themselves by spreading their philosophy or alignment. For example, a chaotic neutral petitioner might "eat" by spreading chaos.

The spirits of the dead reach the Outer Planes through astral conduits. Astral conduits are usually invisible on the Material Plane and the Outer Planes (but visible in the Astral Plane as silvery tubes or whirlwinds). They're generally rooted to specific locations on the first layers of the Outer Planes, and the spirits can be seen emerging as petitioners from those locations, which might appear as caves, pits (filled with maggots in Baator), shining clouds, giant seed pods, or whatever. Petitioners on the shores of Mount Celestia emerge from the Silver Sea (they become lantern archons, so they probably don't need to breathe - they're just shining globes of light).

It depends on the plane. In the Lower Planes, petitioners usually start out as larvae - wormlike creatures with tortured human faces - and slowly evolve into manes, nupperibos, or hordlings, though there are some petitioners in the Lower Planes who look like they did in life. In the Beastlands, petitioners look like animals, in Limbo they look like swirly blobs of chaos, on Mount Celestia they're lantern archons, in Pandemonium they're clawed, emaciated humanoids who are streamlined to protect themselves against the wind.

If they look like they did in life, I normally imagine them as still seeming a bit eerie, with glowing eyes and vacant expressions. Their bodies may be partly luminescent, and even translucent. Whatever they look like, they're dead, and they interact with the living like ghosts do. They're visible ghosts, and you can usually touch them and talk to them, but they've shed their mortal passions and desires in favor of a desire to become one with their plane or god. A Bytopian petitioner doesn't care about the things it did in life - eating, sex, becoming famous or loved, having fun - it cares about personifying Bytopia's traits of industry, community, and cooperation. Living people should feel a bit creeped out by petitioners, since they don't act like people anymore. Petitioners may act as servants to the living or to more powerful planar beings, since their chief motivation is to impress their plane or deity with their good behavior (however they interpret "good"). They may spend every second of their days singing praises to their deity. In the Beastlands, they act like animals, hunting or being hunted, devouring their prey or being devoured by their prey (and, surely, being reborn the next day to do it again). They can talk, and they may know ancient wisdom, but they care little about sharing it with the living.

Of course, there will always be exceptions. Some petitioners may not have completely shed their mortal desires, and they may try to integrate themselves into living communities or even pass as living creatures. I like the idea that they never quite fit in, though - there's something a little off that people who pay attention can pick up on.