B737 Undifferentiated or Other Model
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Type 282
Situational Awareness; Confusion
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors
During descent and arrival my Captain and I were discussing some of the potential trouble spots going into SNA. This being my first time into SNA we were discussing the particulars of the short field and also discussing the airspeed restrictions under the Class B airspace. When loading the flight plan and arrival we discussed the RNAV RNP Z 20R as the expected runway. We briefed up the approach and planned to conduct it. The Class B discussion was a distraction [as well as checking for] the lack of any discontinuities in the flight plan; [and] we both missed that we had not been cleared from the approach. At the IAF (KLEVR) the aircraft was in LNAV and VNAV PTH and full automation. As the plane began the right turn and descent on the approach we queried Approach Control to make sure that we were cleared for the approach. They cleared us for the approach immediately but they were expecting us to continue on the heading at the bottom of the STAR and then they were to vector us to final.
B737 First Officer reported a track deviation occurred on arrival into SNA due to turning before being cleared for the approach.