The English idiomatic phrase, “get in on the act,” or alternatively, "get in on the action," is one that is pretty easy to understand; getting in on the act just means becoming involved in something when it is profitable. This somewhat literal phrase tends to translate fairly easily to English beginners. It is often used to express some business opportunity or similar situation.
The origin of the phrase, “get in on the act,” is generally not well defined. This phrase developed as a way to illustrate someone finding an opportunity and seizing it. Here, the word “act,” a word classically reserved for theatrical performance, is applied to a real-world situations in a colloquial way. Serious business situations or other parts of human society are often referred to as an “act,” "action," or, in other idiomatic constructions, a “game.”
English speakers also have other alternate phrases for the idea of seizing opportunity. Another common one is the phrase “get in on the ground floor.” This phrase relies on the metaphor of an elevator. The idea that a business opportunity might experience rapid growth is contrasted to the idea of a physical elevator rising from the ground floor to the higher floors of the building. In this allegory, the speaker wants to “get into the elevator” before it “rises,’ or in other words, get involved in a business opportunity while the cost of involvement is still low, with a lot of potential for profit later on.
In addition to the above, even more English idioms focus on the idea of opportunity or similar concepts. For example, the more enigmatic phrase, “get a leg up,” has often been used by native English speakers to illustrate getting an advantage, often getting an advantage over someone else, but sometimes just for a general positive change in someone’s overall situation. This phrase relates to the traditional activity of horse racing, where a horse receiving medical care might be, “getting a leg up.”
Another detail about the expression, “get in on the act,” is that it assumes that others are already initiating something. This phrase is usually used as one “insider” talking to another. For example, someone who professes to know about some secret opportunity may say to someone else “I’ll give you a chance to get in on the act” where the listener envisions a variety of other people already cleverly profiting from what is being described. In many situations, someone using the phrase may be in fact painting a less than honest picture of an opportunity, which is why it is important for those who receive these kinds of proposals not to rely on the words of another person, but to research the opportunity themselves.