Even when switched to dynamic mode, my characters do not bend and stay really straight

Either the joint limits are not configured correctly (open your character file in the character maker and check physics properties), or the force is really applied the same way on the whole body. In this later case, you can used body masks on the Force Behavior in order to get your characters to achieve different effects (e.g. apply force only on left part of body to get a rolling effect, or only on the head for a headshot...).