Opportunity Knocks was an American television game show that aired on ABC from 1973 to 1988. The series was known for its unique format, which combined elements of a traditional game show with reality television, and its focus on the lives of its contestants.

The show, hosted by the affable Jim Lange for the most part of its run, followed a simple yet effective format. Each episode invited a contestant to come onstage and play a game that was customized around their lives. This approach earned Opportunity Knocks a reputation for being one of the first reality TV shows ever, long before the term even came into existence.

The structure of the show involved three segments or 'turns'. In the first segment, a film crew would descend onto the home of an 'ordinary' American, unbeknownst to the selected contestant. The crew would document their lives, capturing their daily routines, hobbies, interests, and interviewing friends and neighbors for interesting tidbits about the contestant’s life.

In the second segment, the contestant was invited to the studio. Onstage, they were faced with a game of questions—with the twist being that all the questions were based on their own lives. The answers, hence, were not only points-earning responses but also surprising personal revelations.

The final segment was named 'The Prize Stunt Round', where contestants could win money and prizes. This round included physical challenges related to their hobbies or lifestyle, filmed earlier. For instance, if a contestant was a fan of baking, they might have to identify different types of cakes blindfolded.

Interaction between the host and contestants was one of the greatest highlights of the show. Jim Lange’s warm, humorous demeanor set the tone for the show, and his empathetic, engaging interview style made contestants feel comfortable, adding to the show’s appeal. He was later replaced by Geoff Edwards and Vera Jimenez in the later years.

The prizes ranged from cash awards to unique, personalized prizes, which were often humorous and in line with the theme of personalized quiz questions. This could be anything from a lifetime supply of a contestant's favorite snacks to all-expense-paid vacations which were often destinations previously expressed as 'dream vacations' by the contestants.

One might wonder whose opportunity was knocking through the show. Was it the opportunity for the contestants to win exciting prizes, or was it for the audiences to peer into the real lives of 'ordinary' Americans? In essence, it was both. Opportunity Knocks not only served as an entertaining game show but also acted as a candid archive of American social life during the time it was aired.

The show was known for its humanizing portrait of contestants in the era where game shows were typically marked by impersonal quiz questions and generic prizes. The customized structure of Opportunity Knocks brought a breath of fresh air to the game show landscape during the 70s and 80s, contributing to its long-running success on ABC.

Over the span of 15 years, Opportunity Knocks continued to present a fascinating sociological lens into American households, subtly shifting the dynamics of television as it bridged the gap between reality and game shows. Even while the show ended back in the late 80s, its legacy continues to influence contemporary television.

In summary, Opportunity Knocks was a unique blend of reality television and game show, featuring a format that added a personalized touch to every single episode. It showcased the daily lives of ordinary Americans, presented exciting games based on contestants' personal details, and rewarded them with imitable prizes. High on entertainment, humor, suspense, and surprise, Opportunity Knocks was certainly a jewel in the crown of American television history during its run on ABC.