It's a good question and it probably has even more roles. So I would suggest you discuss/examine all the possibilities where it might be involved - prime mover, stabilizer, synergistic accessory muscle etc.

Consider synergistic rolls with the sartorious & rectus femoris (which may be different in the later depending on what the knee is also doing at the time e.g. extending), the roll played both concentrically & eccentrically from a flexed position e.g. a sit up/roll down to the floor from long sitting. Also it's action from a lengthened position such as hip extension vs's it's action at 90 degrees hip flexion. now that may be different if the pelvis is 'fixed' or supported as in sitting on a chair vs. a state of unsupported movement e.g a pike position in diving. The discussions are so varied and the cases for examination are therefore such that you cannot generalise the data from a single action that has been examined in a piece of research to all actions of the iliopsoas and for that matter the iliacus affect as well. It does make for a nice forum discussion though.