If your partner opens with a "pass," does that make you the "captain" in a bridge partnership?
You ask three or four different things here; and while Mycroft does a good job of discussing the concept, I wanted to directly answer your questions.

Are you the &quot;captain&quot;?
No, you're not the &quot;captain&quot; necessarily, if you mean the person who ends up deciding where you go.  Captaincy largely means one person has a well-described hand, and the other has a more wide open hand; &quot;pass&quot; describes a huge number of hands in first or second seat.  Sometimes you do end up captain - sometimes partner does; if you open 1NT, for example, partner is nearly always now &quot;captain&quot;.
Can you mislead your partner? No, you should not try to mislead your partner.  You should, in every situation, make bids that are in keeping with your agreements (and convention card).  Those agreements can make accommodations here though - &quot;Could be light 3rd/4th seat&quot;, or as Cohen suggests, open 4 card majors 3rd/4th seats. Those are agreements - they're not misleading anyone, hopefully.
Can you take liberties? By agreement, you can do things you wouldn't do in 1st/2nd seat, and those things do include being a bit more likely to make an opening bid.  Does that mean you should make bids that your partner doesn't expect or doesn't know what they mean? No, absolutely not.  It just means the range of things you can do is greater - by agreement.

Third or fourth seat openings can still lead to big problems - partner could have a solid 11 count with 4 in your suit and be just short of opening 1st/2nd seat, but then they see you opening and think &quot;Game!&quot; when you really only have 10 yourself and 4 cards as well.  Partner isn't passing permanently!  I've seen people who do make inconsistent calls in 3rd or 4th seat, and it rarely works in their favor - even really quite good players capable of winning regional-level pairs events.