The phrase "fell in" can refer to a variety of situations, including sudden emotions or physical accidents. Synonyms for "fell in" could include "plunged into," "immersed in," "engaged in," "sunk into," or "captivated by." Other options might include "overcome with," "enamored by," "enthralled in," or "lost in." Depending on the context, synonyms for "fell in" could also include phrases like "dived headfirst," "tumbled into," "was enveloped by," or "was suddenly overcome with." Whatever the situation, there are plenty of ways to describe the idea of "falling in" with more specific language or imagery.