I think the main thing you’re looking for is “differentiation”. Unicellular organisms are generally homogenous, as each must be adapted to surviving on its own.

I think the main thing you’re looking for is “differentiation”. Unicellular organisms are generally homogenous, as each must be adapted to surviving on its own. Simple multicellular organisms are often just masses of individual cells. But, as you start increasing the number of cells, it becomes evolutionarily advantageous for some of them to take on more specialized roles, rather than have every one be capable of everything. That is differentiation. As these organisms get more complex and develop more and more differentiated cells, mechanisms also evolve to prohibit cells from “regressing” to undifferentiated forms, and to tightly control aspects of cell growth. Unlike unicellular organisms, a functioning multicellular organism needs to have all its cells coordinate in order to survive. Human cells for instance contain a regulation mechanism which detects abnormal behaviour and directs the cell to commit suicide (a key protein in this mechanism, p53, is widely studied in cancer research). So no, your cells don’t get their own “will”. They do what they are genetically fated to do. Those that don’t either die, or wind up as a cancer that chokes the life out of their host.