If your actors has a single 'update' method, the first actor would check for collisions and move before any other actors had a chance to do anything. This gives certain actors preferential treatment in movement.
Of course, you could instead plan out the dependencies better; do all collision detection before all physics response. But this is just a different way to look at 'immutability'
There may be exceptions to this; it is not a subject I keep current with.
foreach actor(find collisions -> find response -> do movement)
foreach actor(find collisions) ->
foreach actor(find response) ->
foreach actor(do movement)