“Second acting” (or “second-acting”) a Broadway show is to wait for intermission, go in the theater with the crowd, sit in an empty seat and see the second act for free. A person who does this is a “second actor.”
   
“Second acting” may be as old as Broadway, and there is testimony that it dates to as least the 1950s. “A Gaining Sport: ‘Second-Acting’: Some Theatergoers Make It a Point to Miss First Acts (and Box Offices)” by Marjorie Rubin, was published in the New York (NY) Times on July 1, 1963.
   
         
Wiktionary: second-act 
Verb
second-act (third-person singular simple present second-acts, present participle second-acting, simple past and past participle second-acted
1. To sneak into a venue during the intermission without paying.
 
New York Show Tickets
Second Acting at a Broadway Show
(...)
Second Acting Was Popularized over Thirty Years Ago
Especially popular in the ‘70s and ‘80s, second acting was a way of getting the Broadway show experience without actually paying the high ticket prices. The idea would be to hang out with the smoking crowd in front of the theater during intermission, then mix in with the herd as they headed back inside. Once in, the second act-er would scout out an empty seat that was either unsold or abandoned by someone who decided to leave during intermission.
 
Sneaking Into A Broadway Show For Free
It sounds easy enough, but those trying to second act have often found out the hard way that an empty seat isn’t always empty. It can be quite an embarrassing situation when the seat’s occupant arrives (perhaps delayed by the long lines in the restrooms or at the bar) and the usher has to get involved. If you get caught, second acting will get you kicked out of the theatre in a heartbeat.
 
1 July 1963, New York (NY) Times, “A Gaining Sport: ‘Second-Acting’: Some Theatergoers Make It a Point to Miss First Acts (and Box Offices)” by Marjorie Rubin, pg. 31, col. 8:
“Second-acting,” as old as the Broadway theater itself, has become such an “in” thing as of late that certain ultra-sophisticates consider it a point of honor to be unable to discuss the beginning of a play.
 
The technique is to mill with the intermission crowds in front of a theater and then walk in with the returning ticket-holders.
 
If a show is not selling out, the second-actor can usually slip into an empty seat undetected by an usher; if the show is a hit, he can join the standees, hoping that the ticket stubs won’t be checked.
(...)
Musicals are the most likely Broadway fare for second-actors, but straight plays are given a whirl, too, In fact, there is a small coterie whose members gain a wry literary satisfaction from guessing what happened in the first act.
 
Google Books
So Nice Not to See You:
A Comedy for Seven Women
By Fred Carmichael
New York, NY: Samuel French
1968
Pg. 20:
LAURA. I never knew how people could do that—that—what do you call it?
FLORENCE. Second-acting.
(...)
Pg. 21:
FLORENCE. You have to appear as if you have one (a ticket—ed.). First, you pick up a program in the lobby and then, just before they ring the buzzer for the second act, go down to the ladies room. After everyone is seated, come bustling up and sit in an vacant seat. It’s always best to read the first act before you go or you miss a lot of the fun. You know, I haven’t seen a whole play since 1953.
 
Google News Archive
24 July 1978, The Village Voice (New York, NY), “Second Acting: Half the Shows on Broadway, Free” by Samuel Dennis, pg. 49, col. 4:
Want to see half the shows on Broadway without ever buying a ticket? Half of each show, that is. “Second acting” is one of the Great White Way’s oldest, least known traditions. All you have to do is wait for intermission, walk in, and see the show. It’s that easy. It’s also cheaper and shorter, and the best part of most shows is usually the second act.
 
Google Groups: rec.arts.theatre.musicals
Do You Second Act Broadway Shows?
Feuillade
2/9/99
Second Acting (or sneaking into Broadway shows at Intermission to watch the Second Act) is practically a Broadway tradition.
 
I’m writing an article on the subject (for Talkin Broadway), and I’m looking for people who do this on a regular basis.  Confidentiality is assured, and no real names will be used in the article.
       
Google Groups: rec.music.opera
second-acting
Deborah Overes
5/12/99
JDavis6627 wrote in message
news:19990511173330.16220.00002123@ng139.aol.com…
> >Has anyone ever second-acted an opera?
>
> Not really sure what you mean by this, but when the Met gave Rimsky-Korsakov’s
> COQ D’OR in the ‘40s, the ballet acted out the story while the singers stood at
> the sides and sang the score.  Is this what you meant?
 
Second-acting is sneaking into the second act of a show by mingling with the smokers who’ve come outside at the intermission and re-entering the theatre with them.  I haven’t personally done this and don’t know of anyone who has. Interesting question.
 
Google Books
In and Out in Hollywood:
A Novel
By Ben Patrick Johnson
Richmond, VA: Palari Pub.
2002
Pg. 99:
“Well, go tonight! Second act it.”
 
“Pardon?”
 
“Sorry — starving New York actor term. Second acting is when you walk up to the crowd out front at intermission and sneak in. The ushers don’t usually check tickets then.”
   
Google Groups: rec.arts.theatre.musicals
Can “Second Act” Still Be Done? 
Dan the Man
5/29/03
.(JavaScript must be enabled to view this email address) (JULIAN) wrote in message news:<25038-3ED…@storefull-2338.public.lawson.webtv.net>

...
> With all security issues after 9/11, can “second act” be done?
   
On top of 9/11, you would think that the Chechnyan terrorist episode in Moscow last year would have effected a shutdown of second-acting in NY.  But it still goes on unchallenged as ever.
         
Broadway World
SECOND ACTING - Is it still done?#2
capnkidd
Posted: 10/6/12 at 5:35pm
Yes, thogh it isn’t as common as it used to be, in my opinion.
 
New York (NY) Times
A Lost Art on Broadway: Sneaking In for Act 2
By JANE H. FURSE SEPT. 23, 2016
(...)
There was a time when “second-acting” — sneaking into a Broadway theater at intermission before the second act — was as common as the cigarette break in the middle of a musical. It was a time-honored rite of passage, practiced by generations of starving actors and students of the theater. It required a confident air, a visible copy of Playbill and the belief that somewhere there would be a free seat. But today, when security is ultravigilant and shows are under pressure to sell out night after night, the practice has all but gone dark.
(...)
Broadway luminaries such as Carol Burnett, Charles Nelson Reilly, Marian Seldes and Elaine Stritch have shared in memoirs and on film their second-acting escapades going back to the 1950s.
 
A Way with Words
Second-Acting
Posted by Grant Barrett on April 3, 2017
Second-acting, the once-common practice of sneaking in to see the second act of a Broadway show for free by mixing in with paying patrons outside at intermission, largely ended as theaters began tightening their security and fewer people step outside for a cigarette. This is part of a complete episode.