One time on a Delta jumpseat 737-800 from SFO-DTW redeye, the CA was PF and was cool as **** but the FO was trying to run things. Example was climbing and in clouds and the temp was +9 and anti ice was on. It goes to +10 and the second it hits +11 the FO says ok here it comes off. CA said well, lets just wait I think the temp will come down again. FO does it anyway. Reaches and turns it off. Then the temp goes below 10 and the CA just calmly says see? FO turns it back on. That was just a small example. By the time the leg was done I thought if I hadn't seen the CA on the yoke/throttles for takeoff/landing, I would have thought it was the FOs leg
FO was a ASA guy who had upgraded relatively quickly back in the day and was hired at Delta at a decent young age. On that leg he was being a right seat CA. Might be okay for one leg, but would be annoying over a 4-day trip.