There are a variety of synonyms for the phrase "acting as", depending on the context in which it is being used. Some possible alternatives include "serving as", "functioning as", "fulfilling the role of", "playing the part of", "assuming the position of", and "taking on the responsibility of". These phrases all convey the idea that someone or something is fulfilling a specific role or function in a particular situation or context. Choosing the right synonym for "acting as" can add nuance and clarity to your writing, helping you to convey your ideas more effectively and precisely.