Given scene/plot descriptions from movies or TV shows, and given a specific question about the scene, summarize the question into a title

Q: I have always wondered what techniques directors use to get amateur actors, especially young ones, to cry. How can someone surrounded with so many film crew and cameras can concentrate enough and feel so sad that s/he can possibly burst into tears? In general, is crying in a movie scene (with real tears) something that can be learned by practice or is it something that only talented people can perform?

A: What techniques are used by directors to make an actor cry?
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Q: In Prometheus, for what reason did Weyland hide on the ship? When he finally revealed that he was in the ship, it turns out nothing happens, there are no particular consequences and the reason for the secrecy is unclear. Why didn't Weyland just inform the crew that he's also aboard in the ship? Can someone explain to me, what are the reason for this?

A: Why does Weyland have to hide that he was on the ship?
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Q: I've never understood how Kirk, Spock, and Bones got back to the present at the end of "City on the Edge of Forever". Going back to 1930 is simple enough, they stepped through the portal at the right moment, but there was no portal on their end! How did they get back? Finding an answer is proving to be difficult because even after trying numerous search-term permutations, I only managed to find a single mention of the discrepancy. I can understand if maybe they wanted to focus on the emotion and stuff of the episode, but it's a science-fiction show, and this was one of the first incidents of time-travel in the whole franchise, so getting back is a fairly big issue that can't just be glossed over.

A:
How did they get back in "City on the Edge of Forever"?
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