B737 Captain reported as they taxied onto the runway for departure the Captain noticed the First Officer set the flaps to the wrong position and a complete Before Take Off briefing had not been accomplished. The Captain reported the First Officer made many mistakes and lacked proficiency due to being a new student who had not flown for a long period of time.

Narrative

I was the Captain on Flight XXX ZZZ-ZZZ1 performing Line Check Airmen (LCA) duties with a new hire on date. This was an all-night flight which was leg five and a third duty period for the new hire. My new hire was Military background and had completed his OE early this year and basically sat for four months. He had been back for a three-day sim recently. He has a great attitude and is eager to learn; however the first few legs he was challenged with a lack of -121 experience and what I believe was a degradation or loss of good habit patterns from the sim phase. Leg three on day two was a short; challenging flight from ZZZ2-ZZZ3. He was stressed and challenged making many mistakes; but ultimately I could see improvement; despite being overwhelmed with the pace of our operations. Leg four was a long leg from ZZZ3-ZZZ that same day; and showed significant improvement. The approach and landing were approaching 'line standard' and I was hopeful if not confident that we had gotten over the 'hump'. We had a long layover to ready for the all night flight back to ZZZ1.I had stated that typically; it's much easier to fly from ZZZ to ZZZ1; so we could have a good flight and finish required briefings. The day before Flight XXX I had two periods with naps; early afternoon and then again in the evening before report time. My student reported that he had napped; but said he wish he had drank more coffee when we were in the cockpit. When I awoke an hour prior to report; the operations flight plan was not present; so I readied myself to the duty day. In the 10-15 minutes prior to leaving the room there was still no operations flight plan; but I had checked the weather along the tracks as well as ZZZ1 (all good; and VFR). The hotel van was late by almost 10 minutes. We arrived to chaos at the gate area. ZZZ station was buying seven passengers off the full flight. I pulled up the operations flight plan and immediately called Dispatch to remove fuel and get the latest ZZZ4. I then directed my First Officer (FO) to the cockpit to get ready for the flight; telling him that he was responsible for the FMC and that I would do the exterior walk around to give him additional time. Dispatch; Station and Flight Attendant conversations dominated the next several minutes and I arrived at the cockpit from the walk around approx. 25 min prior to pushback. He had most of the programming done and it was all accurate. He had been slow but usually pretty accurate in previous flights.We discussed the upcoming flap 25; Bleeds Off take-off and I had him pull up the supplemental procedure and review it. We were able to get all the passengers on; and we briefed and did the preflight checklist. I had briefed that we would push back; start both engines and run the before takeoff checklist to completion before taxi. Additionally; he had briefed a flap 25 bleeds off; and we analyzed the data and discussed the winds (080/10) and temp (23C) as these were critical to taking the planned weight. The door closed and we worked a clearance with ZZZ5. My FO was seemingly having issues with radio calls; attempting to take calls for other aircraft. So I slowed him down and we received our clearance and expected altitude. Then we pushed back and I directed him to start both engines as briefed. The preflight had been hectic; with me being pulled in many directions. I had shielded him from as much of that as possible; by having him simply work on his cockpit set up. I am very aware that my error signature is when I am rushed; I make mistakes. As such; when the brakes are released; I stop rushing and slow down. I briefed my student the same thing and I make sure to slow my speech pattern and checklist response time.That is not unique to this day; it's every time. I had briefed him that we were going to take our time and we were not rushed. I have no memory or perception for us rushing after push this night. After the number two engine was started; he was sitting there; and I saw the packs were off. This was a common error he had had in previous legs; so I pointed to the packs. The pushback crew was at my ten o'clock position and I looked back at them. Then I glanced in and saw that the number one engine wasn't started and I verbalized 'Start Number One'. He responded; 'Oh you want me to start Number One? OK.' I then turn my attention back to the crew as it was time for the salute. I look back in and the engine is at max motoring; at approx. 34 percent N2 with the Fuel Control Switch still in Cutoff. My FO mumbled something about 30 percent which I didn't hear. I asked him to repeat it and he did. It was something about a 'limit'; so I asked again and got a reply I didn't understand; so I slowed down again and said; 'I do not understand what you are telling me'. He was confused; and it turns out that he was mixing up previous airplane's limits.I prompted him to start the engine. We ran through the After Start Checklist with no issues that I'm aware of. No without moving the aircraft we began the before take-off checklist. The first step is 'Final Weights' from the ACARS printout. The number he read made no sense. I was thinking about it as he went on; trying to understand when he backed up and corrected himself to a number that made more sense. I'm assuming a transpose error. Then he went on to the takeoff weights/speeds and really had troubles. He was going back and forth from the takeoff data message to the FMC. I stopped him; and we talked about him analyzing the data; and checking it with the FMC prior to the checklist initiation. He understood; and I stopped the before takeoff checklist and we started again from the beginning. I remember the first two steps clearly. I remember the trim step clearly. I remember the FMC step clearly as we had discussed the routing that was absent a SID. In the later moments and the hours since I have no memory of mention of flaps. We taxied to the runway; ensured the bleed panel was correctly configured including techniques; got the release from ZZZ5 and took the runway. As we were taking the runway; I was going to go over the flap 25 takeoff again and talk rotation rates and pitches and as I said; 'OK...a Flaps 25 takeoff...' and as I glanced at the gauge; the flaps were set to flaps 5. He reselected flaps 25; stating 'You probably said 25...' We then stopped on the runway and accomplished the before takeoff check in its entirety.I know the 'whys'. I don't know the 'how' and it'll be a while to cement the 'what am I going to do next'. He's new; with no airline background. He sat for four months. In my opinion he's lost habit patterns and confidence. There were operational pressures out of the station; along with numerous distractions. I knew and briefed that we were rushed and that we would take a moment. We did. I slowed things down. We both had stated that we napped; but it was late; XAAM for him; XBAM for me. The single most disturbing thing I'm wrestling with it for 20+ years in the B737; I've been very careful with flaps. From day one in the left seat when we get to the step; I pause (so that I can clear any taxi conflicts and go heads down) point to the FOs Take-off Page in the FMC; then point to the flap gage. Then I reach over and wiggle the flap handle in the detent. I couldn't in the moment; nor can I understand how I would have missed the step in the checklist or worse; how I could have possible stated '25 indicated' without a visual conformation and a tactile one. And I have no memory of any part of that step.My best guess; which considering my actions that day are suspect; is that he became increasingly overwhelmed and as his errors started occurring; he didn't have the habit patterns or muscle memory or experience to dig himself out. He's new. He sat an unconscionable amount of time and is trying as hard has he can. But he told me that night; 'Chair flying can only go so far' and he's right. He can't possibly recreate the distractions and unanticipated threats. I knew he was overwhelmed; he had been the whole trip; but the previous leg was so much better; maybe I have a bias that he was 'over the hump'. During engine start; I knew he was making errors; but I thought I was slowing down and trying to create an environment where he could catch himself up. And ultimately; I blame myself. Because in trying to be an above average instructor; I became a very substandard Captain. In discussing this with my best friend from military instructing days; himself a Captain at another carrier; we tried to find a 'what will we do next'. He suggested that I make a rule that when errors start piling up; stop instructing and just be a Captain. I liked it and vowed to add that to my personal checklist until I woke up this morning with the realization that had I done that; and not started to instruct him on the mechanics of the flaps 25 takeoff; I might not have glanced at the gauge. I might not have caught it. So I'm back to square one.