Once the wires were approved I would start the UI process in earnest even though I may have started already in the wire stage. With short sprint cycles any edge you can get the the previous stage helps. I start to apply brand and finishing style via comps.
Time will determine if I work out click through prototypes or something more interactive via, inVision, Sketch or Axure. On this project Illustrator click through PDF’s where the standard and worked well!