If you are talking about a profession someone is in the field. A chemist is in the field of chemistry. For a location, if you are physically standing in the middle of the field, ironically you are on the field. Where is the coach? He's on the field, running toward the goal!

Those prepositions being used in the sentences are both correct. 1. She is in the field. This means that the she is in the place referred to as the field. 2. I am lying on the field. This means that the you are lying on the surface of the field. it's like I am lying on the bed. you can also learn "in the field" as an idiomatic expression. Example:When someone's in the field, they're "in direct contact with a source of data or subject of interest," as in doing work outside an office or laboratory. It can also mean "in actual use," like a product release after R&D, or it can have the more literal sense of "within a given profession or area of expertise." hope it helps you.