What are bidding sequences in which the responder is likely to be the "captain" of the partnership?
In addition to Opener's notrump openings, whenever Opener rebids notrump to show one of the gap-ranges in the opening notrump structure (typically after opening one of a suit or 1C/2C strong) Responder becomes Captain by virtue of a better grasp of combined partnership holdings. 

Likewise if Opener makes a splinter raise of Responder's response then Responder becomes Captain.

Either Opener or Responder also becomes Captain by making a Blackwood or Gerber Ace-asking call, an area where beginners and intermediates often err by having the wrong hand makes this call.

Also Responder becomes Captain, at least temporarily, by making a Fourth-Suit Forcing (FSF) call at the second round. Consequently you will see this call made by experts on a three card suit from time-to-time as a prepared rebid, often revealing on the third round a hand too strong for an immediate splinter raise, which would have given Captaincy to Opener without properly revealing Responder's strength).

It's important to note that Captaincy can change during the auction, and these situations should be well discussed by any ongoing partnership as they are rife with possibilities for gross over- and under-bidding. Even in world-class play it is not unheard of to see slam played in part-score, which is a sure sign that a Captaincy change error occurred. It is especially important to discuss how and when these occur in competitive auctions.