The following reasons I am about to outline are not specifically directed at you but just trying to give an idea why a Senior Captain may question you or doubt you more.
* Your experience on the a/c type - if you have low hours.
* Your experience with the airport - if its your first time landing there.
* His experience with the airport - he may be familiar with the airport and its STARS and never heard such a clearance given before.
* His experience flying with you - have you flown together before.
* His perception of you - are you quiet, introvert, against his more dominant character.
* As mentioned in above post - ego
At the end of the day there is no excuse if one pilot is not sure or if ever you come across a situation where you question something ALWAYS get confirmation from ATC and also applies with Cabin Crew and other personal e.g ground, such as re-fuller or dispatcher.
As a Senior Captain he should know better, but atleast he admitted his fault and apologised which is commendable. What you need to do is think how can 'I' learn from this and how could I have avoided it... hence making you a better airman.
The fact that you have already reviewed your actions is excellent... Like you said perhaps it would have been prudent to keep the Captain in the loop by telling and confirming with him what your entering into the FMS. Also rather than going into explanations as soon as he was not in agreement (a warning sign), tell him directly to ask ATC to confirm the clearance.
Also think if the situation had been different and ATC never noticed your 90 degree turn, what would you have done then?...(thinking point)... If there was high terrain or conflicting traffic things could have turned out more serious... as he took control and was now PF that makes you PNF and in charge of the radio, so now you can call ATC and get confirmation.
These are all learning points and if you go away and think about how you can do things better or create scenarios where things may be different it will benefit you greatly.
I'm not having a go at you at all, I'm totally on your side but you will always fly with different captains and you will always come across these different issues, hence giving you more experience.
Aka.
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If there's any doubt then there's no doubt!